The best way to get you started is to have a spotter - someone to help you out. Have the spotter lift you up when you can't do any more and you come down by yourself. Gradually lessen your reliance on the spotter.
When you can go past 15-20 pullups you may want to do some variations. One of those variations I do these days is the pull up on suspension rings as you see me doing below.
So throw in some pullups at least twice a week into your workouts and reap the rewards.