Respect the elderly, protect the innocent and be a mentor to at least one kid in your life.
Dont do drugs, stay outta trouble, stay AWAY from those that MAKE trouble and always consider the consequences now instead of later.
The girl you dump today may be the next Nobel prize winner tomorrow. You dont have to treat her like a queen, but at least when she's 40 she wont include you as a mistake or "that loser" on the Tonight show either.
Always go with your first gut instinct. If that little voice in your head tells you something is wrong or something doesnt feel right... dont do it, no matter what the pressure is. The person that is trying to influence you probably wouldn't do it HIMSELF either... and he wont be the person that will stick around and explain why you're in a coma to mom and dad later on.
Learn a trade, clean your room, change your underware, and learn how to cook. All those things will win you the girl you've always been looking for, later in life.
If the military is for you, go for it! If not, then go to college. There will be no better memories than those during your military stint or that time in college. Either way, it's best to leave town. Go around the world or at the least in another state. That will be the best time to test your state of independance and help to prepare you for things to come.
All those times you saw dad under the car while you were running out the door to go hang with your friends.... turn around, go back and ask him if he needs a hand or at the least give em a hug. You never know when you will one day walk past where he used to be and wish that he were there to do that then.
You are not invincible and think about the days ahead. I know, it's already been said... but it's the truth. No one ever thinks for tomorrow, but just the here and now. Everything you do now will sooner or later come back to either bite you in the ass or help pat you on the back. It's your choice. Make the right decisions today that will help you tomorrow. Watch your spending, don't get loans you have problems repaying, don't drive like a maniac, tell someone you're sorry or you appreciate them, get educated, learn how to change the oil or install some parts, don't do drugs, don't drink and drive, etc. All those things you did as a teenager can affect you later in life - getting a loan for a house, car, job or career and a person that you truly love.