I've been donating pretty much since I turned 17, which is when you can start here. My high school would get people every 3 months in a group and go donate, and I've just kept with it since then. I gave at the start of October, so I have another donation scheduled for December 3rd.
I've really found the best thing to do is go in a group, with friends or the like. It is a great way to spend some time with friends, eat some killer snacks, and save up to three lives. Sounds like a good deal to me. I generally get faint after every donation because I'm pretty small in stature, but it only lasts 15 seconds, and then in 3 months I'm right back in the chair. So although some people do have minor issues, if it isn't a serious health problem for you, it's a good thing to do, and you feel really good about it after.
I'm O+. Anybody can take my blood, and I give often.
Isn't it O- that can give to everyone, because it has no markers, then O+ can give to everyone but O-? B to B or AB, and A to A or AB. That was my understanding. But O+ is the most common, and second most universal. Still really good to have a lot of! Tis my type.