^{(The content in this Instructable first appeared in my new blog at http://biodieselhauling.blogspot.com/
To my 40 Instructables subscribers, and anyone who stumbles across this and likes it, be sure to take a look, as much of what I write isn't quite right for an Instructable.)}

Bikes are perhaps the greatest human invention of all time.
They get you from where you are to where you want to between 2 and 5 times as fast as you could walk there, but they use absolutely no fossil fuel or external energy what-so-ever, which means they have basically no environmental impact and no operating costs beyond the original manufacture and purchase.

However, if you are not already a "bike person", the amount of choices in type and brand and size and accessories can be overwhelming. If you buy new from a local bike shop, the salesperson will most likely walk you through the process of deciding what will fit your needs, but if you want to save money (and further reduce your eco-footprint) you should really buy everything you possible can used.

So, for the beginners who know just about nothing about bikes but want to get one from Craigslist, or perhaps a thriftstore or yardsale, I'm going to break down for you exactly what to look for and what to avoid.

Step 1: About Me

First, for anyone who doesn't know me personally, my credentials on the subject:

I began riding regularly for fun and transportation in 1992, when I was 12. The next year I began riding to school every day, so that I could keep the bus money for other things. In high school, in addition to daily commuting (to school and internship) and weekend rides of 40-100 miles, I began annual 4 day trips down the CA coast with a group of teachers and friends. After college I went with the couple that had organized those annual rides from San Francisco to Puerto Vallarta Mexico, and went solo from there along the coast to Acapulco and then North to Mexico city (over an 8000ft pass) for a total of 2800 miles over 2 months. When I returned, I took a job as a bicycle messenger. I eventually ended up also working as a messenger in New York City.

In 20 years of serious riding, I have had a bmx bike, a steel touring bike, a British internal-hub drop-frame from the late 60s, a carbon fiber racing bike, an aluminum mountain bike, and two folding bikes, all of which together I paid a grand total of $450 for (of which $400 was the carbon fiber road bike).

Eventually I returned to CA where, for the past 5 years, my primary job has been as a hauler (mover, and handyman) which involves picking stuff up that people don't want anymore, and then finding new owners for those things. This involves either selling or giving away anything which is still useable (which is most of what I pick up), frequently on Craigslist.
My second job for the past 5 years has been as a mechanic in a tiny bike-shop of sorts, the Bike Station, whose primary service is FREE secure valet attended bicycle parking, but also offers relatively low-cost repairs. Because we don't sell new bikes, and because we never turn anyone away for lack of bike quality, I have been able to work on a great variety of bikes, of all types and ages and cost levels, which is rare in any one shop.
(My third job is a reserve for the Coast Guard, but that isn't relevant to this at all)

And now... on to the content!

Step 2: The Single Most Important Thing to Know When Buying Any Bike (new or Used)

DO NOT BUY A DEPARTMENT STORE / TOY STORE BIKE!!!!!!!!!!!!!!!!!!!!!!

Ever. Don't even consider it. Seriously. Not even for your kid. They are absolute garbage. They shouldn't be legal to ride on public streets. They should be considered toys at best. This includes pretty much anything you can buy at WalMart, Target, or Toys'R'Us, or whatever local equivalent you may have where you live.

The most common brands are Magna and Next (both Dynacraft company), Pacific, Roadmaster, Huffy and Murray (all Dorel Company), . One of these bikes brand new is worth less than a real bike that is 30 years old. (Actually, my favorite bike, the one that I rode to Mexico and used a messenger is almost 40 years old).
Less common brands include Anza, Ozone, Rhino, Vertical, Malibu, Avigo and Sonoma (all owned by Dynacraft) Dyno, InStep and Powerlite (all Dorel/Pacific)

Only slightly better are most modern Mongoose, Schwinn, Ironhorse and GT. All of these made decent bikes in the 80s and 90s, but were then bought by Dorel and quality dropped. Not all of these bikes are department store quality - my own mountain bike is a GT - but if you aren't confident at telling the difference, it may be safer to avoid all of these brands (or find a friend who can). The same company also owns Cannondale, but unlike with their other brands, Cannondale never made department store bikes. Schwinn has recently begun producing decent entry level bikes again as of 2014, but they had about a ten year run of only making department store quality bikes.

I can not emphasize this enough. Having worked on these many many times, I can say with absolute certainty that they are not worth the price if you are given one for free. They use the cheapest parts that can be found, parts which have been obsolete for decades in the rest of the bike world, and those parts are put together by people who know literally nothing about bike mechanics. I have rarely seen one, even brand new, that had everything adjusted properly. They are so cheaply made, and so poorly assembled, that they are dangerous.

*I have already gotten a couple of comments suggesting I must be a "bike snob" to say this. Not so. All of my bikes have been cheap. I have never been a racer. The shop I worked in was a non-profit community bike shop, and we never turned anyone away, no matter what they rode. That's how I got to see so many department store bikes up close and from the inside, as well as test riding them (after repairs).
These bikes are not suitable for casual riding, and not just because they are heavy. They are built with the cheapest parts possible, and then poorly assembled. By riding one you are automatically increasing your chances of getting into a crash.
From a mechanics perspective, the poorly built parts translates to additional labor for repairs (which they need more often) which in turn means higher repair costs - often higher than the original purchase price. For this reason, many bike shops will not even work on these bikes.

The best clue that the bike you are looking at is a department store bike is if it has a one piece crank.

See pictures above for examples

Another clue that the bike you are considering is a rolling pile of crap is brakes that look like the ones in the pictures, called simple caliper brakes.

Note that most road bikes will have compound caliper brakes, which are similar in shape, but slightly more complex. The keys to the crappy brakes are the rectangle brake pads and the flat metal brake arms, with just one single pivot point in the center.

Unfortunately, many department store bikes less than 5 years old often have slightly better components - 3 piece cranks and direct pull brakes - so it is slightly more challenging to recognize them at a glance, other than by brand.

Step 3: Type of Bike

Like in everything else in life, every choice you make is a series of tradeoffs. There are a lot of different kinds of bikes, that are good for different kinds of things. You can use pretty much any bike for anything, but some will make the trip a lot easier.
Presumably most people reading this is looking for something practical to commute on, (and not a racing bike or a bike to do tricks on)
Some questions to ask yourself are:

Will I want to carry a lot of stuff with me?
Will I want to go fast sometimes? (I found I am often running late, and ended up using my racing bike to commute pretty often)
Are the roads well paved, and/or will I ever want to venture on trails?
Will I have to climb any steep hills?
Will I ever ride in rain/snow?
How important is comfort?
How important is efficiency?
Will I be working up to high mileage sometime in the future?

If you want to carry alot, consider touring bikes or something meant specifically for commuting or hauling (of course, you can always retrofit any bike with a trailer, so that's another option)
If you want to go fast, and you don't need to carry much with you, a road-sport / racing bike will help you get there quicker with much less effort
A road-sport or racing bike, however, does poorly off-road, or with major potholes, gravel, glass, and curb-jumping
For steep hills, no matter what kind of bike, you will want not only gears, but low gears - generally that means 3 chainrings (the 3 big gears in the front, attached to the crank), like in the picture above.
Some bikes - especially ones specifically intended for casual riders or for commuting have the gears hidden inside the wheel. This has the advantage of being cleaner and simpler for the user, because all moving parts are hidden. They are also less prone to being broken and are more weather resistant for the same reason.
The trade off is that they are more expensive (to buy and to repair), heavier, and the lowest gear of an internal system will not be as low as with external gears. For this reason, if you have to ride up steep hills, go with external gears. If you ride mostly flat or only small hills, its a matter of preference.

For inclement weather, a mountain bike, hybrid, or purpose built commuting bike outperforms any road bike, (although you can put fenders and lights on anything, you can't put wide tires on a racing bike).
Just like with a Cadillac and a Geo Metro, there is a trade off between comfort and efficiency. Beginners often get a bike with a wide saddle and tall handle bars, so they can sit up-right like a big rolling sofa, and then get discouraged when all the cool kids on real bikes pass them by at twice their speed everyday. Its not just cause they are weak - there is a reason racers have low handlebars: the biggest factor in bicycle efficiency is wind resistance, and when you are upright, your torso is like a big parachute. So that comfort comes at the cost of twice as much effort to go the same speed. On the same note, a mid-range hybrid or mountain bike can weigh literally twice as much as a high-end road bike, which is a lot of extra pounds to lug with you up the hill to your house at the end of the day.

None of that is to say don't buy the hybrid, just understand that there are trade-offs. They are trying to be decent at everything at once, without costing much and making new riders comfortable, which means they excel at nothing.
The best compromise, in my opinion, between efficiency, bad road/trail worthiness, and bad weather capability, is a cyclo-cross bike, which is a road bike meant to be used off-road, however they are somewhat hard to find, and therefor usually expensive.
The next best all-around option, (again, in my personal opinion) which also includes cargo hauling capacity built in, is a road touring bike (which would be why that's been my primary bike for the last 2 decades).
But that isn't to say that you can have a perfectly good long-term commuting experience with a mountain bike, racing bike, hybrid, cruiser, or a 50 year old bike from before special purpose bikes existed.

If its possible, I'd recommend seeing if you have any friends with various types of bikes you can borrow for at least one ride, to see how they differ in riding position, gearing, handling, and so on.
If nothing else, maybe you can find a local bike shop and do a test ride on a couple of different bikes. You may be taking advantage of them even more in the next step (since you will ultimately buy used directly from someone), but to make up for wasting their time, plan on buying all your accessories (minimum: helmet, lights, gloves, bell) from them, and/or having them do a basic check/tune of your new (used) bike once you get it.

Step 4: Fit

The only way to really get it right is to go to a bike shop and find someone who really knows all the variables, and has access to an adjustable bike fit machine (a stationary bike where you can change the lengths and angles of all the components and frame parts). (see picture above)

They generally don't charge for this, but again, if you go for a bike fitting, and don't buy a bike from that shop, you should really buy your accessories and/or service from them. Bike shops operate with a pretty small margin.

As a super quick and simple rule of thumb, when you stand over a bike frame (in front of the saddle) with your shoes on and feet flat on the ground, there should be at least 1 inch, preferably 3 or 4, between the top bar and the beginning of your... special parts. For one thing, if you were to be in a minor crash and fall forward off the saddle, you don't want to be impacting a big steel (or aluminum, or carbon) bar of bike frame. This is also an indication that the frame more or less fits you. If, when you straddle the bike, the top tube is actually touching you, that bike is much too big for you, and no matter how much you love it and no matter how good the price is, it just isn't the bike for you. I'm sorry.
On the opposite end, you can use the seatpost to judge if a bike is too big.
When you ride, your knees should be 99% of the way straight at the bottom of each pedal stroke (not 100%, or locked-out, but almost). If you don't extend your legs all the way, (imagine how it looks when an adult rides a tricycle meant for kids), you will end up hurting your knees.

The seat post (the part that attaches the saddle to the frame) should have a line on it marking the maximum its meant to be extended (it may or may not say words to that effect). By loosing a screw or bolt at the place where the frame clamps the post, you can raise or lower the seat height. If the post is at its highest (at the line) and when you ride the bike your legs are not extending fully, then the bike is too small for you. You can always buy a slightly longer seatpost if its close, but if the seat needs to be higher than about 10 inches above the frame (assuming the top tube of the bike frame is horizontal, more on that soon), chances are the rest of the dimensions are too small for you anyway.

Of course, many bikes don't have a straight horizontal top tube running from the handlebars to the seat. The sloping top-tube or the frame that used to indicate "girl's bike" (in order that the bike could be ridden in a skirt or dress) has become common on mountain bikes and commute oriented bikes and many hybrids for both genders, as well as continuing to be common for female specific road bikes. With a sloping top tube you can't just stand over it and measure the distance between your body and the frame to determine fit, and the seat-post will have to be extra long. In that case you just draw an imaginary line (or better yet, use a level and some string or a broomstick or something) where the top tube would be if it went straight across.
(see picture)

There is really a lot more to it than that, (such as the length of the top tube and the angle of the seat tube) but to keep things simple, 1-5 inches between you and the bike frame (or where the frame would be if it were straight) with your feet flat on the ground is about the best approximation there is.

Once you find one bike that fits, you can check the frame size (usually printed on the seat tube) and have a rough idea of what other bikes will also fit you - generally mountain bikes and hybrids will be measured in inches, road bikes in centimeters. But be aware that the numbers can vary between bike styles, brands, ages and depending on whether the top tube is horizontal or not.

So now you have some idea of what kind of bike you are looking for, and what size it needs to be.

Step 5: Frame and Components

There are hundreds of brands, with even more choices in components, so it is impossible to recommend any specifics, or break down all of the possible combinations. They all have high-end and low-end stuff, so never assume a bike is good or bad just based on the brand of the frame or the components.

Older bikes can be very good quality, just try to stay on this side of the 80s, when most manufacturers switched from carbon steel or high-tensile (Hi-Ten) steel to chromoly (ChroMo) steel. The only modern bikes that still use carbon steel or hi-ten are the department store bikes, but pre-80s they were the standard for all but the most high-end bikes.

Some of the more high end older bikes did have good steel frames, particularly those made of Reynolds 531 or Columbus steel.
If you can find one, in good condition, you are looking at a good frame even though it is older.
However, unless the prior owner kept up with upgrades throughout the years, you should still consider a newer bike, as many advancements in components have been made since then.

Some brands which are in department stores today were once legitimate bike companies (most notable is Schwinn: a 10-20 year old Schwinn is likely a good bike, while a 2010 Schwinn is probably from a department store -better than a Murry or Huffy, but still not a good choice)

If it has a sticker (usually on the seat tube, near the frame size sticker) that tells frame material, avoid high tensile steel. Cromo or 531 steel, aluminum, titanium, and carbon fiber are all good frame materials, with their own pluses and minuses, but for practical commuting purposes, you should mostly be looking for steel. Aluminum is also decent, and usually a little lighter. I prefer steel for a practical commute style bike, because it is the strongest of all frame materials, and it can be bent back after a crash and keep going strong. Any other frame type, once damaged or bent, is no longer safe to ride. However, if you find a bike that is perfect in every other way, don't necessarily shun it just because it is made of a more exotic material.

Suspension adds weight, complexity, and cost, and unless you ride off-road, doesn't serve much purpose, but if you do ride-off road, (or have giant and frequent pot-holes to contend with), it can make the ride more comfortable and give you better control. In general, for a commute bike, I would recommend skipping it.

Skinny, high pressure, tires have less rolling resistance and less weight, therefore you can go faster / further with the same amount of effort. However they have less grip in bad conditions (esp snow), give a bumpier ride, and are (in theory**) slightly more prone to flats.

** In my personal experience, I have not had any more flats on my road bikes.

The curved style of handlebar found on road bikes gives you more possible hand positions, which can be nice on long rides once your hands get tired, but ultimately handlebar shape is a matter of personal preference. If possible, try a couple different styles out on the open road before becoming committed to any one type.

I would recommend an adjustable height and/or angle stem for any new cyclist, so you can choose whether to be more upright (more comfortable, but higher wind resistance) or more low and sleek, or anywhere between and change between them whenever you want, as you ride more and find what you like. Most older bikes (10 years or more) have an adjustable height stem standard. Most modern bikes have a "threadless headset" (commonly called "Ahead-set", which is actually a brand name) which are not adjustable in height, but you can attach an adjustable angle stem to it
(see pictures)

As long as this already is, I am deliberately leaving out gobs of information, because this is a basic guide, and because you are buying something used and have limited choices. You aren't custom ordering a $3000 bike, so if you have a hard-core bike fanatic friend, you can safely ignore most of what she tells you about frame butting, spoke counts, gear ratios, or the superiority of Campy parts over Ultegra and Dura-Ace. Remind her that your goal is to get from one place to another, not to win a criterium. Tell her you don't even know what a criterium is.

Step 6: Craigslist

In case you didn't already know, Craigslist is awesome. It is one of the most effective tools combating the worst aspects of the American consumerism cycle in which we are constantly throwing away things that still work in order to buy a slightly newer version, wasting our own money along with massive amounts of natural resources. Craigslist helps recirculate our money in our own communities instead of having it siphoned off by giant soulless corporations, keeps good stuff out of landfill, and prevents material extraction and energy consumption to create new stuff, all by the simple act of bringing together people who have stuff they don't want with people who want stuff they don't have.

If you are lucky enough to live in a reasonably large city, with at least a moderate amount of cyclists, than chances are good you can find a large selection of high quality bicycles on your local Craigslist page.

If not, though, the same principals apply to buying from a yard sale or flea market.
Another option is a used bikeshop. They will generally charge a bit more than buying directly from the previous owner, but you get the advantage of a bike that has been checked out by a mechanic, and a knowledgeable person to help you with the selection process.

Expect to spend at least $100, but unless you are lucky, probably $200 or more.. Yep, that much, for a used bike. Even an old one, or a simple one. Less than that, and the chances are good you are buying a department store bike, a 40 year old bike which was low quality when it was new, or a bike that has been left out in the rain and generally not taken care of . Spending less will likely mean that you have to spend much more in parts and service in order to get it up to decent operating condition. If a bike is decent quality, well taken care of, and severely underpriced, it may well be stolen. Don't support bike thieves. They will end up stealing the bike back from you again. That's called Karma.
A good new bike is generally $700 and up, so you are still saving plenty of cash by spending 200. Of course you can always get lucky and find someone who has no idea what they have is actually worth (like I did with my $400 carbon fiber bike), but don't expect it.

At the same time, be aware that there are a great many over-priced bikes on Craigslist. Depending on your market, it may take some patients to find a good one. Do a Google search of the make and model of any bike you are considering. In general, look for bikes no more than 20 years old, and of course, avoid department store bikes like the plague.

So say you have read this far, determined what you need, figured out the size, and browsed around and found a few ads that seem to have good quality bikes.
Keep in mind that someone is selling the bike for a reason. More often than not, the reason is because they just never ride it. Which usually means it hasn't had any maintenance in... ever. If its been stored indoors where it is dry, that may not be a problem, but in a damp basement or a back porch, there is likely internal rust that you can't see. If you see rust on the outside, unless the person is a bike enthusiast that you trust maintained the bike, assume there is internal rust as well. A bike bought by someone who never got around to riding it is more likely to have been cheap to begin with and to not have had much care taken in its purchase.
These are obviously not hard and fast rules, but if they are selling due to a move out of state, or due to having several other bikes, the chances are a little better the bike was taken care of.

Take hold of the wheels, and try to move them side to side (perpendicular to the way they turn). There shouldn't be any play in them side-to-side, against the axle. Do the same for the cranks (what the pedal attaches to) and the handlebars, trying to wobble them side-to-side, not the way they are supposed to turn. If they are loose and wobbly, that is a sign the bike hasn't been taken care of. Those things are easily tightened (although some require specialized bike tools) but if they have been ridden loose, they are likely to need more extensive repairs.
Spin the wheels while holding the bike off the ground, and watch how the rim moves relative to the brake pads. If the rim moves so much that it hits the brakes on every revolution, this is usually repairable (by "truing" the wheel - adjusting the tightness of each spoke) but it is another sign of a bike that wasn't maintained.

Avoid a bike with any obvious dents or cracks anywhere in the frame. If a bike has been in a severe crash, there may be more damage than meets the eye. I've had a frame suddenly break on me. It isn't pretty.

Tires and handlebar tape can be replaced, and a bad paint job can discourage theft, so try not to let the aesthetic elements overwhelm the mechanical ones.

Expect a test ride, but offer to leave something with the seller (like your ID) so they know you don't ride off with it, since it is essentially its own built-in getaway vehicle.

Once you have (FINALLY) found a good bike that fits your needs, its time to make the deal.
Because this is Craigslist, don't forget you can try to negotiate.
Bring cash (it lets the seller know you aren't a scammer), and show up in person (but bring a friend who knows bikes if you have one)

Always make a receipt (preferably make it in advance with blanks for the bike and seller info). It should have the make and model of the bike (both are usually written somewhere on the frame), and as much descriptive information - color, frame type, size, age, any notable features - as possible. It should also say the purchase price, date, and the buyer and sellers names. And above all, it absolutely should have the bikes serial number, which will usually be stamped into the metal somewhere on the underside of the bike (you have to turn it over to see it), most often on the bottom bracket (the thing the crank axle goes through)
If the person refuses to provide their name, or seems hesitant to have you check the serial number, there is a good chance the bike is stolen, and you should politely move on.
Having a receipt will also be useful in recovering the bike should it ever get stolen from you, or in having insurance replace it if it isn't recovered.

Step 7: Accessories

Now that you have it... time to spend a little more money
Its a good idea to have it checked out by a qualified mechanic, or at least a friend who is a bike nut. This is a perfect opportunity to pay back the local bike shop that helped you out with advice while you were shopping.

If there isn't a local bike shop around that you owe a favor to, Bike Nashbar offers some of the lowest prices you can find anywhere on bike accessories.

While you are there, you need, at a bare minimum, a new helmet. Don't use a helmet that came with the bike, or any other used helmet. They get internal damage from absorbing the energy in a crash, which may not be visible from the outside (it is basically a bundle of microscopic bubble wrap, and in an impact, the little air bubbles pop). Therefor, a bike helmet is one-time use only. If it is used, there is no way to be sure it hasn't been in a crash already. Even if it hasn't been in a crash, repeated drops, even just from 4ft up to the ground, can gradually wear away its impact protection, as can UV exposure from the sun and ozone exposure from traffic.
If you ride at night, a headlight is absolutely essential, along with the reflectors that come standard (or reflective tape if someone removed the standard reflectors). Even better is an extra bright headlight, a tail light, the standard reflectors AND reflective tape. Run the headlight even when it is light out anytime there is reduced visibility, such as when there is sun glare at dusk and dawn, or on an overcast day. I've noticed I can see cyclists better when I am driving when they leave their headlights on in daytime, therefor I now run my lights ANY time I'm on the bike, day or night, even when its clear out.
Getting hit by cars is not fun.

Other very good things to have are gloves, which absorb road bumps and protect your hands in a fall, and a bell to warn pedestrians without having to yell.

Some people swear by cycling shorts, which have a pad built in to supposedly make sitting more comfortable. I have never cared for them. More important is to get a saddle that fits you properly. It should be slightly wider than the distance between your "sit" bones - you can feel them protruding at the bottom of your pelvic bone when you squat. measure that distance (preferably at home alone) and look for a saddle no more than an inch or two wider than that (and definitely no narrower than that). Having that distance correct will make more of a difference than any cut-outs or gel inserts or shock absorbers or any other gizmos and "features" you find on modern saddles. The saddle should be firm. This will feel more uncomfortable at first, but as you get used to it, and you put more miles on the bike, it will be more comfortable (and do less potential damage to important body parts) than a squishy saddle. The reason is because you sink down into a soft saddle, and it puts pressure on all parts equally, while a hard saddle supports the sit bones only, keeping everything else above it, like it should. I know the big wide cushy gel seat seems like riding an easy chair. You just have to trust me.
The reason I put this under the accessories section, and not the components, is because saddle selection varies so widely, both by gender and among individuals, that you shouldn't expect to find a bike that has a comfortable saddle for you already in place. You might get lucky, but don't reject an otherwise good bike because the seat is uncomfortable. If possible, try to find a shop that will let you trade, if the one on your bike is in good shape, but just not right for you.

Step 8: One Last Thing

7) One last thing
A bicycle is considered a vehicle, and as such, they are required to follow the same traffic laws that cars do. Ride on the right side of the street. Stay off of the sidewalk (unless expressly allowed in a particular location). Stop at red lights. Stop at stop signs***. Ride predictably and signal your turns when warranted. The majority of bike/car collisions are partially or entirely the cyclists' fault. Two of the most common causes of crashes are bike riders riding on the sidewalk, and bike riders riding the wrong way (on the left side of the road). Another common cause is lack of visibility on the part of the cyclist. Eliminating these few (totally controllable) factors actually makes riding a bike statistically safer than driving a car. The thing most new cyclists worry about - getting clipped from the rear by passing cars - is actually relatively rare. Crashes happen primarily at driveways and intersections, and they happen because the cyclist was somewhere the driver didn't expect them to be.

If you are in the SF Bay Area, consider taking the FREE traffic safety course sponsored by the local Bicycle Coalitions: http://www.ebbc.org/safety
If not, check with your local shops, riding clubs, or bicycle coalition to see if anyone offers something similar.

UPDATE: I just wrote a new post specifically for new riders who aren't used to being in traffic, to help you avoid getting hit by a car.
This post has been way more popular than I ever expected, and since it is intended for new riders, I thought it would be pretty important to help y'all not only pick out a new bike, but not get run over while you are riding it!
Read this before you get on the road: http://biodieselhauling.blogspot.com/2012/06/please-ride-your-bike-in-street.html

*** I won't pretend I stop at stop signs, or even try to convince you to. But at least slow down for them, and look both ways before you cross. And always come to a full stop if there is cross traffic which has the legal right of way.

[Someone has written an article with counterpoints to this one. Personally, I disagree with him on a few points (1st off, that you should never buy a used bike!), but it is always worth getting 2nd opinions and different perspectives: http://hiawathacyclery.blogspot.com/2012/01/bike-buyers-guide-for-beginners.html ]