I taught myself guitar mostly from tab in magazines and books from the library. The best way to start is to come up with a list of songs that you really like (you'll be playing them over and over while trying to figure the guitar out . . . so you better damned well like them) and ask someone who plays guitar to identify the easiest ones (or at least easiest riffs). Then just jump in.
Once you're comfortable fretting some single notes and getting the picking so that you can time them well while you play along with a recording it's time to start learning chords. You're going to start with open chords first (E, Am, D, G, C). If you're most dudes in college, this is where you stop too . . . because you open up a whole world of songs that you can play on an acoustic and sing to. And chicks dig a guy who can sing while strumming a guitar. Allegedly.
If you're unlucky enough to be a mediocre singer, after that you're going to work on barre chords and scales. If you're into punk/metal/hard rock you will learn power chords and lose your shit entirely. Scale work progresses typically from minor pentatonic (if you're a blues fan you can almost stop here once you've figured out string bending - maybe throw in some mixolydian and dorian licks and you're good) -> major pentatonic -> major/minor scales -> modes -> exotic scales.
If you're still attempting to progress beyond this point you are likely branching into:
- A metal lead guitarist: Tapping, whammy bar action, pinch harmonics, and maybe some ripping off of classical music. You either already like, or will develop a strange fascination for the phrygian mode.
- A jazz guitarist: Chord building, chord inversions, diminshed scales, chord melody. You enjoy improvising over a steady and unending stream of chord changes. A pop guitarist plays three chords to a thousand people every night . . . a jazz player plays a thousand chords to three people every night.
- A prog rock guitarist: You hate 4/4 time and songs that clock in at under 25 minutes. Every other person in your band is a middle aged dude. Your drummer has a kit with 300 pieces that takes nine hours to set up, so you do most of your band practice at his place.
- A classical guitarist: You've got freaky long fingernails, a near superhuman ability to site read, decent composition skills, and typically can't improvise for crap.