Re: I cannot even do 1 pull up. What muscles should I train to improve this?
Could try using an assisted machine if you have access to one. If you go down that route though, I would only use them for as long as it takes for you to do one unassisted. You see some people at the gym doing sets of assisted pull-ups or dips every session, and they're obviously using it as a comfort blanket. Especially with the dips. I refuse to believe that any adult with a fully operational set of testicles can't do a single unassisted dip after months of training.
If you can do "half" pull-ups unassisted, try doing them to build up your grip strength and confidence.