I'll tell ya how I got into good pull-ups. I started with the kind that fit over a doorway. It's easier and more secure-feeling than the high bars. Nowadays I like to try the high pull-up bars at the gym, which I have to extend my full body length to reach. I can only get a few reps on those successively, but I'm getting better.

Also, my gym has this machine which forces the user to reach up, grab the handles, and pull down (in a seated position). I dunno the name of that machine, sorry. :o The resistance is variable. btw, when you do any sort of "pulling" exercise, drive with the elbows to get the best results and minimize exhaustion.