Really, the question is not what you would say to your 13-year-old self if given the chance, but how you would make yourself listen. You know what you were like at 13: a whiny, opinionated, entitled brat, even if you were hiding it. What makes you think that unstable bag of hormones would listen to anyone, even if it's you? Unless you're talking about parties, music, dating, sex or maybe food, or how much it sucks to be 13, you're just going to be a buzzing background noise to the rest of the world thundering in your own young ears.
Mokalus of Borg
PS - And that kind of renders your time travel a bit pointless, doesn't it?
PPS - I hate when that happens.