Well, I'm 23 now, so I'll skip back to 16 or 17 (because of a late birthday I turned 18 during my first year of college).
First and foremost, more than absolutely anything else, I'd tell myself to enjoy it. I worried about a lot of crap in high school and now, after being out in the rest of the world, I don't know, I wish I'd...valued it more. I wish I'd taken the time to be a kid. I'd have gone to more parties, tried to make more friends.
I spent three years of high school and longer out of it chasing around a girl I thought was someone special. Or, who I wanted, really wanted to be someone special is probably more accurate. She wasn't, and it was nothing but a waste and a lot of heartache over nothing. God, I wish I could do that over. That's one of my biggest regrets, and if I had the power to go back and tell myself at age 16 just what the next give years were going to be like, I'd do it in a heartbeat.
It's strange, but thinking back on high school, on being 16, I really do wish I could do it over again. I know it's not great to dwell on things we can't change and all that, but more than just about anything, I wish I could be 16 again.