Let's be honest, You can't do enough chin ups. Neither could I.
But let us start at the beginning. You want to gain muscle and You want to gain it fast, right?
One of the best back builders is the simple chin up, I say simple, but everyone who tried to do 12 chins with perfect form knows, it ain't that simple. But it doesn't stop at the back, Your arms will grow noticeably. That was one of the things I couldn't believe, when I started doing weighted chins my arms got huge.
Now the good part is You can train chin ups nearly everywhere, be it that low hanging branch in the garden or one of those door mounted chin up bars.
Important is, start early, start now!
The first one is the hardes, trust me, I've been there. But the feeling after doing it, You may have just started training, or lost a lot of weight. The feeling is just incredible!
I won't write a program here here there are plenty online for You to read. But I will recommend one I used to bring up my chin ups to a solid 20 reps.
The program which helped me the most is right HERE. Free to view.