I have almost no upper body strength, what can I do to build it up?

Practice, practice, practice. It may seem that you can't do anything, so what you need to do is learn to modify the exercises to fit what you're able to accomplish. You don't want to make the exercise easy, but you need to make it so you're actually able to get something done. For instance, trying to do a pull up from dangling when you can't even lift up a bit... that dangling won't help you at all, and straining over and over will only do damage and make you feel defeated. Instead, get a pull up bar and a small stool or chair for support. Do pull ups from the stool, with your ankles crossed and barely any support on your feet. Get a good feel for what you're able to do with some amount of effort and go from there. Gradually as you feel stronger and more confidant, make it more difficult! Just make sure that you're pushing yourself. You can do the same with push ups!