I wasn't able to do a pull up until after high school. It took me to take lifting seriously (and to start practicing them daily) to be able to do them. It wasn't pretty at the beginning but I slowly accumulated more and more repetitions, and therefore more and more pull up strength. Today I am willing to challenge (and confident that I can beat) the majority of the people I come across to a pull-up contest. You can do this too. You just have to get started with a small task and build from there.