I’m 15.
My pitiful words of wisdom are:
Keep your mouth shut around your parents… That way theres nothing for them to twist around and use against you.
Find somewhere where you can goand just swear. swear until you’re hoarse. very good for releasing anger.
stay AWAY form spandex.
Don’t mix alcohol and drugs. You have a hellova headache, and the munchies will kill you.
and finally,
Smile and nod.
make sure you have the reputation you want, not the one you deserve!
I thought of another one; don’t burn bridges. If you do decide to burn some, choose them carefully and make sure you won’t want to go back over them someday. This is especially true of employment bridges.
You don’t have to go see every movie the second it comes out. Most movies are crap, and you’ll save yourself a lot of time and money by waiting a week for word of mouth to get around.
Most important thing I can tell a teenager is this. Most of your friends believe they are smarter than every other human on earth and that they are the center of the universe. The longer you go on believing these things the worse it is for you when reality inevitably boots you in the ass.
Whatever happens, wait a few years and you will realize that however grown up you think you are now, you aren’t. In fact, you may well want to go back in time and give yourself a few swift kicks in the ass. I know I do. This is not a bad thing, it’s just my experience. You’ve got a hell of a lot of learning and growing left to do, but don’t try to rush it. Life will take care of that without you trying to fake it.
And – life really does go on. No matter WHAT happens, you can pick up the pieces and go on. Really.
Two things:
First: You can learn more about somebody by how they treat other people than by how they treat you. If you’re on a date with someone, and they’re nice to you but mean to the waiter/waitress, they will eventually be mean to you once they have what they want. Also, if you get into a relationship with somebody who is cheating on somebody else to secretly be with you, odds are, if you succeed in totally winning the person away from their relationship, you will be the cheated-on person later.
Second: Don’t listen to the advice of older people on the basis that they’re more intelligent than you, because, by and large, they won’t be. However, you should listen to the advice of older people because they have far more practical real-world experience than you do. In many if not most situations, experience is more valuable than intelligence.
Buy good tools.
Oh, one more thing:
Mistakes are to be learned from. We all make them, but we don’t all deal with them the same way. Some of us examine them and see how we can avoid making them in the future. Some of us beat ourselves up over them and shove them out of view, which is an excellent way to set ourselves up to repeat them.
And the person who can learn from somebody else’s mistakes is wise indeed, albeit quite rare.
I’ll second Cervaise’s advice about learning from one’s mistakes - very important. Also:
Know when to ask for help. This applies to schoolwork as well as lifework.
Live and let live. If someone’s doing something that I think is silly but hurts no one, I just shrug & move on. Well, I comment that I don’t get it, then I move on. 
Be as truthful as you can be as often as you can be. You’ll save yourself & others a load of headaches & grief.
Watch it with the alcohol consumption, my friend - praying to the porcelain goddess is no fun. Same thing with the weed. And don’t get into anything harder than weed, if you even get into that.
Do your best to be considerate of others’ feelings, please. Especially wise if you’re living in a dorm/with a roomate/both.
Pick your battles, as well as your friends, wisely.
Pay at least lip service, if not actual respect to those who might be in a position to help you at a later date. IOW, don’t piss someone off unnecessarily.
And last, but certainly not least:
Think twice about doing anything that might end up in you needing bail at the end of the evening. 
“Son, never ever go to a hangin’…”
Good advice my great grandfather gave my father while working in a field.
I always abide by this.
Resist much, obey little!
(:Dworks for me, and I’m still a teenager)
Give back to your community - volunteer time, donate money and/or resources. Hospitals, school, old folks homes, day care centers, or the cranky old man around the corner all appreciate a little help. Read to kids, teach cub scouts how to cook a foil dinner, shovel the old man’s driveway, bring your pet dog to the old folk’s home… you will get more out of it than they will.
Be open to new things and new ways of doing things. Try thai food, go take a scuba demo, learn a new language, take a different route to work or school, have lunch with someone new to your school or from another country. The brain responds like a muscle - you use it, and it will grow and stay strong. Let it sit idle, and it will atrophy.
And always, always - preview before submitting to SDMB.
Avoid debt. Seriously. If you have to have a credit card, make it one with a low limit and use it only for emergencies. And pay off any balance you might have ASAP!
Don’t lie, ever, about anything (well, except when the officer asks you if you knew how fast you were going…)
A couple of my dear departed dad’s faves:
Work smarter, not harder.
Some people make their own luck (note - applies equally to good or bad)
Family will always be there.
If someone offers you money, accept it and say “thank you.”
Measure twice, cut once.
Lefty loosey, righty tighty.
Ask girls out. Especially girls who you think are too attractive to ever want to date you. Make yourself be brave, and even when you fail, you’ll be far cooler than if you hadn’t asked at all.
Have lots and lots of sex.
Focus on getting a job you’ll love, rather than a job that pays well. No amount of money can make up for a life spent doing something you hate.