Crawl, walk, run approach to pull-ups
Pull-ups are a great full-body exercise that works your core, upper back, lats, biceps and forearms. As this is a challenging exercise, I recommend starting with hanging on the bars until you gain control of your lower body. Once you can hang, then graduate to a descending pull-up, otherwise known as a negative pull-up.
With a descending pull-up you begin the exercise on the downward phase of a regular pull-up. Have a partner assist you or stand on a bench and start the exercise with your chin above the bar and slowly lower. Once you've mastered the descending pull-up, then you're ready to go full-out, start at the bottom and pull yourself up, and repeat.