There are some training plans out there to get you from "couch to pullup". The trick, essentially, is to start from the top and lower yourself down as slowly as you can. After a while, you start to use resistance bands to reduce the weight you need to lift, and then you start doing assisted pullups. Over time, you drop the resistance until you can do a pullup! It does take some time (~6 week for me to my first good-form pullup).

I'm insanely jealous of the American Ninja Warriors who can do 30 pullups at once. I just don't see how that's possible. But maybe someday I'll get there...