I can't do a full pullup either. A couple of weeks ago I couldn't even really do a chin-up (though I used to be able to). I just did assisted / negatives, which for me means... Jump! Then lower yourself down as slowly as you can. And jump a little bit less every time until you can do it without using your legs at all.

And then once you can do it from standing level, you work up to doing it from a dead hang somehow. I'm hazy on the details there because I've never gotten that far myself.