In reply to ClimberGirl:
A lot of good stuff here. I agree with all of it especially the fingerboards and your young age. Lets not damage yourself before you have gone far.
However, if you only have a pullup bar, you can do variations of Pullups and Chin ups. If you can only do 3 pullups, work on the Forced Negatives (as someone already mentioned) after you have maxed out. And rest a bit between sets.
You could vary the grip, from a Chin Up (Palms towards you) to Pullups (Palms away, and vary the width of the pullup to hit different parts of your back more) for example a wide arm pullup will focus more on your lats. This may sound a bit weird, and I remember when someone first told me, instead of thinking "pulling yourself up" to the bar, "Push" your back up towards the bar.
You can try mixed grip pullups, (one palm away, one palm towards and swap).
Try wrapping a towl over the bar aswell and grip onto the two folded sides with one hand, to work on your grip.
And of course you can do deadhangs.
Obviously, this is just for reference, and from my experience. so take your time, research and be careful with your training. Your still young, and you are still growing, lets not get injured.