This Instructable will be a guide to what you should look for when buying a guitar. I will address all of the different aspects that you should consider so that you can know what kind of guitar will fit you best. It will be directed towards buying an electric guitar so if you are looking for an acoustic this Instructable isn't for you. Also, I know this Instuctable doesn't cover every aspect of guitar shopping but it covers some of the more important parts.
Step 1: A Good Fit for Your Style Of Music
One very important thing that you should consider when choosing a guitar, is the type of music you will be playing. For example, if you play a lot of hard rock or heavy metal, you might want to get a guitar from Jackson, B.C. Rich, ESP or Ibanez. These brands make most of their guitars with thinner necks and lower action (strings lower to the guitar neck). Both of these features allow for faster and easier playing so these are good guitars to shred on. Another example is if you play a lot of blues you might want a Fender Telecaster or a Gibson Les Paul. These guitars are mad with smoother sounding pickup which are perfect for soft rock, blues and jazz. It is also very important the the guitar you choose has the right sort of sound for your preferred genre.
Step 2: Hardware
When choosing a guitar, you have to look at the hardware. Everything from the pickups to the wood used for the body affects how the guitar will sound and feel. Again, some hardware will be better suited for certain styles of music than others. For example, when you play a guitar from brands like B.C. Rich, Ibanez or Schecter, you will find that they have pickups with a harder sound with a lot more gain rather than the smooth sound you get if you played a Fender, Gibson or PRS. If your going to be playing hard rock or metal, brands like Ibanez have pickups that work well for that style of music. The type of bridge is also an important thing to look for. Fixed bridges are much easier to tune but at the same time lack the abilities of a tremolo bridge. Floating tremolo bridges, like ones from Floyd-Rose, add a vibrato bar (sometimes called a Whammy Bar) which give the ability for guitarists to add effects like vibrato or dive bombs by moving the bar. These bridged lock the strings into place so that your guitar doesn't go out of tune when you use the bar. Although this is a nice feature to have these kinds of bridges are VERY hard to tune (I personally have 2 guitars with floating bridges, on Floyd-Rose and the other Edge, and I can say from personal experience that they make it very difficult to tune or replace strings). Some guitars offer bridges with vibrato bars but no locking mechanism. These bridges go out of tune very, very quickly because the strings are not locked into place so if you buy a guitar with one of these bridges, plan on not every using the vibrato bar. The wood used to make the guitar body also makes a big difference in the sound. Low end guitars use cheap wood that doesn't allow for the sound to sustain well. Higher end guitars use wood that helps keep the tone and sustain the sound. Usually when the guitar body feels heavy, it will hold the sound better.
Step 3: Play it Yourself and Compare
One thing I can't stress enough is that YOU SHOULD NOT BUY A GUITAR WITHOUT PLAYING IT FIRST. If you just buy a guitar without playing it or even seeing it in real life you may end up being very disappointed with your purchase. Just because a guitar looks good does not mean it plays well. Go to your local guitar shop and play the guitar for 45 minutes or more (preferably more) until you make sure it feels comfortable to play. 45 minutes might seem like a long time but just playing the guitar for 5 minutes doesn't let you get the full effect of what the feel of the guitar is like. Another very important point is that when you are shopping for a guitar, you should always look at a lot of different makes and models before buying. Even if the first guitar you play seems like the perfect one, make sure that you try other ones out too. After you have found the right guitar and confirmed that it is the one you want then buying it at the store of online fine.
Step 4: Compare
Even after you find a guitar you like, make sure to look at other ones. Although the first guitar you found may seem like the right one for you, if you continue looking you may find something much better for the same price. But you'll never know unless you look.
Step 5: Price Isn't Everything
Just because a guitar is expensive doesn't mean its better than one with a lower price. When shopping for a guitar don't ignore guitars just because they are cheap. Usually the more expensive the guitar is, the better it sounds and plays, but there are a lot of guitars that sound good and are not expensive. Again, the only way to find this out is by trying lots of different guitars.
Step 6: Quality
A very, very, very (did i emphasize that enough?) big part of selecting a guitar is finding one thats made from good quality parts. No matter how good a guitar looks, if it's made from cheap materials it will just fall apart and you'll have wasted you money. The best way to determine the quality of a guitar is just by playing it. If the hardware feels cheap or pieces rattle around, this shows you that this might not be the right guitar. Another very important thing is making sure the neck is straight. A warped or bent neck is a sure sign of a cheap quality guitar. Another way to determine a guitars quality is by looking at customer reviews but I'll talk about the in the next step.
Step 7: Look at the Reviews
Another good thing to do before purchasing a guitar is to look at customer reviews. By doing this you can see what people have to say about the guitar you are considering buying. If the overall reviews and rating are positive then maybe it is a good guitar, but if most people leave negative feedback then maybe you should start looking for another one. Good ways to find customer reviews are by looking at websites like http://www.guitarcenter.com or http://www.musiciansfriend.com/ or by Google searching the model of the guitar followed by the words "customer reviews."
Step 8: A Few More Things to Think About
A few more things you may want to consider are the look of the guitar, the pickups, if it is a hollow or solid body, the price, and the reputation of the manufacturer. Also, if you feel like I have missed any important points say what they are in the comments below and I will add it to this guide.