The pullup is arguably the most difficult Calisthenic exercise. If you are out of shape you likely will not be able to do a single pullup. Unlike most exercises you have to train specifically to do pullups. Pulling your chin up above the bar for the first time is an accomplishment. To get to the point where you can do multiple reps requires discipline, determination, and perseverance.
Read the full article: How To Do 50 Pullups Using The Armstrong Pullup Program