I've just gotten strong enough to do pullups for the first time in my life. Should I stretch before I do them? If so, what stretches?
Doing static stretches before working out is not good. It cools you down and relaxes you, which is the opposite of what you want while you're exerting yourself. What's worse is that by stretching your muscles, you decrease the amount of force that your muscles can exert, and make yourself vulnerable to injuries like muscle pulls.
Static stretches--where you put your muscle into a stretch and hold it there for several seconds or longer--is not a good idea before pull-ups.
Instead, do a thorough warm-up and some dynamic stretches. What I do before pull-ups is from Tom Kurz' Stretching Scientifically. Basically I briefly roll out my joints (wrists, elbows, shoulders, hips, ankles, neck), then move for five minutes. Sometimes I go for a jog, sometimes I jump rope, sometimes I shadowbox. Some people use a stationary bike--it doesn't matter what you do, as long as it's not too strenuous or explosive and it gets your blood pumping. Then I do three sets of arm circles both forward and backward, to make sure my shoulder joint is ready to be stressed. (If my workout includes lower body work, I do leg swings as well for my hips.)