Oh man, lets see... Don't go out with her was a pretty good one. Don't start smoking cigs even though you think it's making you more high. Smoke less weed. Get ready for real life and stop thinking you know anything when you haven't even really started life.
I dunno, I'd wanna go back to freshman year of college to be honest. I could stop myself from partying so much, go to class, not fail out. That sort of thing.