JustinGuitar.com! Really excellent beginner course, took me about six months to go through. I'm now moving into the intermediate stuff, and also branching out on my own into learning classical guitar.

The main key of course is to play and practice a lot. Like any other skill, you really have to put the hours in.

After you've learned the basics on how to fret a note and use a pick, I'd highly recommend Rock Smith. It's basically Guitar Hero/Rock Band, except you plug in a real guitar. It gamifies learning guitar, so it doesn't feel like homework. It doesn't just have you play songs, it also has mini-games that get you to practice specific techniques such as sliding, hammer-ons/pull-offs, string skipping, and proper note fretting.

The difficulty adjusts dynamically. When you first start playing, it doesn't make you play all the notes. Over time, as your scores get better, it introduces more notes until eventually you're playing the song as-written.

If you explore this route, then I'd highly recommend the PC version (Distributed on Steam) over the console version, especially if your home theater setup has any sort of audio latency. I bought the console version at first, and my stereo has about 200 ms of latency, which made it incredibly distracting when I'd strum a note, but I wouldn't hear it until 200ms later. My PC, on the other hand, has zero latency, or at least, the latency is noticeable at all.

Check out JustinGuitar.com - tons of free lessons, songs, etc from beginner to advanced and lots of extra, extremely good paid content as well.