According to Wikipedia, a pull-up is "an upper body compound pulling exercise where the body is suspended by extended arms, then pulled up until the elbows are bent and the head is higher than the hands, utilizing an overhand (pronated) grip".

Negative pull-up

Climb up so you start in the topmost position, then slowly lower yourself. Work against the gravity and try to lower yourself as slow as you can.

Partial pull-up

You can also do a partial pull-up for variety. It's almost like a normal pull-up but you help yourself by jumping. Don't use all your strength in the jump. Only just enough so you can do the pull-up.

Static holds

Static holds can also help to get the first few pull-ups. Check Needmorepower's sample workout.