Over on Jason Nunn's blog, there is an excellent pullup progression setup.
Unlike a lot of others, it addresses a few issues:
Weight. Or, more precisely, body fat. It's hard to expect a pullup when you are hauling a lot of (effectively dead) weight up to the bar. Lean mass - muscle, bone, tendons, etc. - will get you moving, body fat will just make it harder to pull.
Non-Pullup Exercises. This doesn't just focus on pullups, pullups, and more pullups. Other pulling/rowing variations are included to get strength in all directions, which can help you get not only your chin up to and over the bar but in all other pulling related exercises. Strength training of this kind is synergistic, and a pullup might be the goal but strength off the bar is even more valuable.
The progression is here:
So you want to do a pullup? Here’s how.
If you're struggling to get a pullup, or struggling to get more than one or two, take a look.