I agree with a lot that has been said, but I have worked with teenaged girls, and I think the most important thing, that no one has mentioned, is THE talk. I think it should be done in a non threatening way, and as honest as possible (my dad told me if I lead a guy on too much he will get elephantitis of the testicles...whatever). I know this may be hard to acknowledge, but stuff happens, and I would rather your daughter be safe from STD's and potentially dangerous illnessess. I would also teach her that NO means NO and that if the guy does not respect that, than he is not worth her affection. The reason I say this is because sometimes I know what goes on in groups of kids, not to scare you, but there is actually more pressure in the groups now-a-days than often in one-on-one. The guys are more likely to compete with one another, so the talk is IMPERATIVE. Trust me, I have seen what happens when girls are either uneducated, or educated in a very unrealistic way.
Good luck. 13 just starts the ball rolling. . .
L.