Re: I cannot even do 1 pull up. What muscles should I train to improve this?
A pull up is just a particular type of pulling action -- so if you want to be able to do more, find ways to replicate that (or similar) pulling motions. Someone's already mentioned doing pull-downs on a machine. Rows work well, (imagine a reverse bench press -- you'll find ways of making this work) one handed with free weights, or with a barbell.
One thing that might help is to find something to stand on so that your feet are on the ground throughout the pullup, and use your legs as you need to. I often do this to finish off my muscles at the end of a session.