Great topic, here's a few things I'd write to myself:
1. Don't get caught up in the drugs & booze-it's a waste of time
2. Don't stop exercising, you won't stay skinny forever
3. Save yourself from a few years of pain and anguish and stay away from that hot petite lil brunette that you will meet in the club in 1987.
4. Go To College!
5. Last and not least, heavily invest in a couple of little start up companies named Microsoft and Apple...