I'll be honest with you, looking back, I am not sure I'm a huge fan of my 13 year olf self, she was chubby and totally uncool and had terrible dress sense (not a lot has changed to be truthful with you!)
She wouldn't have listened to anyone telling her what to do - but if I had the chance to go back and chat to her, thinking she might listen, I'd probably say not to worry about boys so much, at school they were all dicks anyway. Not to eat so much chocolate because if she did want to worry about boys, the chubbiness wouldn't help her (and maybe I need to take that on board now!) I'd tell her if she REALLY wanted to be a marine biologist, she needed to pay more attention in science class, even if the teacher was a terrible perv! To try harder at the fun sports, and not to worry about the fact running made her sick.
I'd also tell her to stick to her guns and keep wearing those bright red converse because they'd be cool one day!