I wouldn't buy from a pawn shop unless you've shopped new/used somewhere else and really know what something should be priced at. All the pawn shops I've ever looked in want way too much money for abused guitars. I'm sure there are deals out there but you need to be an "educated buyer".
One of the most important things is to find a guitar with a good fit. Everyone has different hand/finger sizes and finding one that fits well in your lap and your fingers fitting well to the frets is v. important. Find a friend's guitar, learn a few basic chords well, and that will at least get you started on knowing what fits and what doesn't. I can wail on my old hand-me-down crappy korean acoustic (broken, not worth fixing) which is about 7/8 the size of my $100 new yamaha beater, which just doesn't fit me well. I play for a couple hours every day and still get hand cramps way too quickly with that thing, not to mention I simply can't play it anywhere near as well as the smaller one.
Regarding the best way to learn, my advice is probably pretty bad. I learned without any kind of chord book or instruction of any kind. Hence if I were to take even basic lessons, I would probably get yelled at for not doing things technically correct. Also, this sounds stupid, but LEARNING HOW TO TUNE the guitar should be one of the first things you spend a lot of time on. You can get very discouraged very fast by not having your guitar in tune.
The main thing I would say about learning guitar (or any musical instrument) is that you should REALLY want to learn. It won't happen overnight. It will take you a really long time to get good. If you walk into the room and cannot help wanting to pick up your guitar just about every spare moment you have, you will probably be ok, regardless of how you teach yourself.