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.