With regards to training your lats (needed the most for your pullups) you really have to concentrate on working them and not just going through the motions.
If you go to a gym get on the lat pull down machine and put a weight on you can just about do 8-10 reps with. Now when you do the reps, make sure you are sitting directly beneath the bar, pull down slowly focusing on getting your lats to do the work. This is hard to do as a beginner but you'll know it's working when you get to about the 5/6 rep and you can fell them straining. You want to be aiming to pull the bar down in a straight line and try to get the bar to finish at the top of your stomach. This is over extending the natural movement you will do on a pullup as you normally just get your chin over the bar, but my pulling the lat bar further down it will help on getting you over the bar when doing a pullup. By far the hardest bit on a pullup (I find) is the last part when you try to get your chin over the bar, this is because your lats are too weak.
One thing I've been trying the past week is one handed pullups. Now before you think I'm superhuman, I'm not, I can do about 5-7 pullups on any given day. What I've found that really helps for me is to grip the pullup bar in the middle with one hand, grab my wrist with the other hand and then try to pull myself up. I can only get about half way up but I really feel my lats working, if you did a few negative versions of these (please be careful) I think these would help tremendously. Again make sure you do them slowly, trying to resist gravity on the way down so to speak.
Oh and finally, another thing you can try to help with your pullups is to use weightlifiting lifting straps (see the link below). You would basically put your hands through these straps and then wrap the remaing part around the bar and then grip the bar. These help because you don't have to rely on the strength of your grip and can focus on working your lats.
http://www.ukfitnesssupplies.co.uk/mall ... G704/55800
Hope it helps (sorry for the long post), but make sure you give it time. Pullups are one of the hardest things to train and get good at.