Some good, and some interesting, advice on this thread!
If I can add anything, it's to try to always hold yourself accountable for your actions and choices. You may feel like a kid at times, but many of your decisions from now on will be subject to scrutiny.
If you have the opportunity to get a college education, value that gift and make the most of it.
When you find yourself in a relationship that seems like the real deal, step back and make sure. It's very easy to lose yourself when you think that you've found "the one," but it's very hard to deal with the consequences when you find out that you were mistaken.
Learn to avoid amassing a huge amount of debt, and if you can, learn to save early.
Enjoy life's little pleasures, and don't pay too much attention to what society, the media, and your peers say you should desire.
Accept immediately that certain decisions can never be undone. One moment of weakness can result in a DUI, pregnancy, infection with a life-threatening STD, or a fatal accident.
Finally, if you set goals, believe in yourself, surround yourself with positive people, and work everyday toward an end, there is very little that you can't achieve.
I'm 32, and most of the above come from either personal experiences or those of friends. Hope this helps!