Don’t break up with Tom, go to college.
More advice to the 18-year-old I was:
At some point during your university or college career, someone will offer you a credit card. Do not take it. Once you start earning money, concentrate on saving it.
Be your own bank. Build a pad of at least a thousand dollars, do not let it disappear, but make loans to yourself from it. But make sure they are loans: pay them back! This pad of money will save you multiple thousands of dollars over the years.
Once you have the willpower to make these savings and pay back these loans to yourself, then you can think about borrowing from banks. Only when you have the willpower and the savings. And even then, don’t give up the savings.
Because if you don’t, you will fall into a spiral of credit-card debt, go through debt counselling, almost declare bankruptcy, spend five threadbare years paying off your debt, and be well on the other side of forty by the time it’s over. Your financial privation will rob you of oppurtunities, rob you of ambition, and further lower the self-esteem that is such a problem, even further imprisioning you in your own mind. You will get through it, but it will take years longer than need be.
And because of all that, you won’t believe you have a chance with someone when that little voice in you says that maybe you do. This will be your second great regret.
Drop out of school…work 28 hours a day…don’t spend a dime…buy as much Microsoft stock as you can. Sell it early 87. Buy it back end of 87.
I would have listened. 
Sell to maintain a modest lifestyle if need to but try not to. Hold on till 2000. Sell all. Live the rest of your life without a care in the world.
If, during this time you meet a woman named Leslie…do not marry her.
Do not, under any circumstances, become a teacher. I mean really…don’t be that stupid.
I really wish I could go back and tell my 18-year-old self, “Don’t be so goddamn full of yourself. You’re a two-bit announcer in a small-town TV station. You have no future here or in this business. You’re basically lazy, star-struck and arrogant. Go join the Navy, learn how to work hard for the benefit of other people, then go to college and become something useful in life, like maybe a high school English teacher. But get the hell out of town, Buster, 'cause if you keep going the way you are, you’ll end up an angry asshole. And nobody likes angry assholes.”
Of course, I wouldn’t listen. But that’s what I’d tell myself.
Travel more often. Go places all your friends are afraid of, alone if you have to.
Lose the weight, hot chicks hate fat guys. When Trisha calls, hang up. If she calls again, rip the phone from the wall. Walk away from Jessica too, and look out for Melissa, trust me. Oh, and skip both the Sara’s, and make sure Dawn doesn’t talk to Laurie.
Stay in New Jersey another week and don’t bother to go to St. Louis. Date the stripper, have lots of sex with the stripper, don’t move her dead ass in.
Sell the truck, buy an old car that you can work on, and keep it forever.
Keep all your guns, and collect the ones you think can’t afford, they’ll be worth it later.
You grew up broke, you’ll have PLENTY of money one day, stop spending like an ass.
Google, Yahoo, Microsoft and Big Oil. Invest until it hurts, but get out when you think you shouldn’t (around 2001) Keep the oil stock until 2007.
Mom’s gonna die early, be prepared for that, and don’t fight so much with her. Let dad know where you stand, and tell him to effin cram it already.
Khasi (the shepherd/husky) is gonna be around 23 years. You should make sure it’s 22. He deserves better.
Those parties, I know you’re gonna live an entire summer without having to work, actually, you’ll live two like that, but it’s not worth it.
Sell the first house sooner.
The fire department isn’t going to pay you back that lost time.
Keep writing and keep reading in public, by the time you get where I am, you’ll be better than you are.
Don’t marry him/her does seem to be a rather common theme for a trip in the ol’ way-back machine, doesn’t it?
Probably none of us would have listened – that’s why they call it “young & dumb.” 
Don’t touch alcohol. Your life will be so much better without it.
The world has shades of grey, your ‘everythings black and white’ speech makes you sound like an ass.
You’re meant to go away to college single.
You have a lot of growing up coming at you, don’t worry so much, it’ll be alright.
Hey, the trip after high school will be an excellent idea. So will actually going to your classes in college. Being a year behind means nothing, so finish the classes, and don’t wait until you’re 27 to decide to go back.
Oh, and buy a house when you first get married. Deciding to wait the year to build a down payment will screw you over as the property values rise much faster than any sort of savings can.
autolycus, great to hear that you overcame those worries about sex. Ive always wondered about that with you. As for me… Im generally happy with my life although i do regret a few choices my 18 year old self would have no context for understanding. I would say that the last year was a complete waste of time but I did learn some life lessons. However it only took a few months and not a whole year!
Now for my 18 year old self. Quit trying to be so cool and dump your “cool” friends. Its not who you are and they don’t respect you. One day you will have loads of self confidence and you’ll feel so stupid for letting these people make you feel bad. Also you underestimate your character.
Graduating early is a good thing. Concentrate on college, however, rather than High School. Otherwise you’l stumble around in college, wasting your parents money and losing your scholarship.
Don’t take that summer job that seems perfect; it’s a farce. All they want is cheap labor because temps are more expensive than naive college kids.
Save your money! Stop spending! You’l blow through so much without anything to show for it!
That depression that you have in check right now is going to cause you so much grief in a year. Just let go of your pride and see a therapist, it’l make life so much easier.
Stay away from the redhead.
Buy that '57 Strat you saw for $1500.
Don’t. Just don’t.
You’re not as bad as you think you are. Honestly. You’re also not as good as you think you are, either. Develop some perspective.
Go to college. Dad is not going to appreciate your staying home to care for him; he wanted to die thinking that he had been successful in sending at least one kid to college. Get over what he says to you when you stay home anyway; he doesn’t have much time left, he’s in pain and on heavy drugs. Forgive him; he loves you as much as he’s capable of loving.
Respect yourself enough to try more difficult things instead of making the easy choices.
Life is not XXXXXXX High School. It will get better.
February 10, 1989, you are going to be approached by a heavyset red-haired girl at work who will clearly shows she has the hots for you. No matter how she comes onto you, do not fall for her suductive charm. Yes, I know you’re still reeling from the wounds of your recent breakup, but if you want a girlfriend, keep looking. No, you don’t want anything to do with her, trust me!
March 1993, a friend of yours is going to talk you into attending a “get rich quick” seminar. They are going to feed you lies and all kinds of glamorous-sounding bullshit to reel you into their scheme. Oh sure, they will offer temptingly alluring testimonies and fantastic tales of going from rags-to-riches and attaining instant wealth virtually overnight. Oh, and it’s soooooo easy! All it takes is a few hours of your time, and you can work right out of your home! It will all look good on paper, but when you see it on the paper that is your bank statement, it won’t seem like such a terrific deal and the only people getting rich are the scamsters who are promoting the program. Run away from this one; run far, far, away!
July 2005. Watch out for a guy at work named Curtis. He’s an asshole and he’ll cost you your job. Avoid him!