Graduation is a time when people you don't know will step forward and give you slightly embarrassing inspirational advice. When I graduated, 10 years ago, the world told the kids of my generation to wear sunscreen.
It's good advice, really: you should wear sunscreen. You should also probably follow a lot of the directions in that now-legendary editorial/Baz Luhrmann song adaptation: if you can get through the Oprahisms like "dance, even if you have nowhere to do it," there's some pretty solid advice in there. You're going to hear a lot of "words of wisdom" from people who barely remember the good things they've done but can recall their mistakes with crystal clear detail, you're going to be told to avoid living your life like they did, or maybe you'll be told to live your life exactly as they did, as if their youth and the adventures they had were the ultimate in living and something that everyone should go though. Basically, everyone on earth who wishes they had a chance to perhaps go back and give the years 18-22 another spin will be giving you not so much advice as a play-by-play on how their daydreams have unfolded over the past 15-20 years. I'm pretty sure this happens because it's hard to even put into words how fast the world moves once you leave high school, and how different things can be, even after only 10 years.
On the day of my high school graduation, in June of 1999, Bill Clinton was the President of the United States, Britney Spears was dating Justin Timberlake, half of my graduating class had signed up for the Marines because it was an easy way to get money to pay for college and nobody thought we had a shot in hell of going to war, gas was 99 cents a gallon, people were all excited about this new music sharing thing called Napster, the first iBook had just come out, and half the world was pretty convinced that the apocalypse was coming in approximately 6 months due to the y2k bug. I went off to college thinking I'd leave with a job just handed to me, unless the world exploded on December 31. Good times.
That quickly changed however: since my graduation there were the September 11, 2001 terrorist attacks on the United States, many of those Marines I went to school with have since done several tours of duty in Iraq or Afghanistan, gas was up to 4.40 a gallon around here last summer, the economy has tanked, unemployment levels are up, and George W. Bush was elected president—twice. There have been highlights as well: I don't think anyone would have predicted the election of President Barack Obama when I was in my cap and gown. Technology has rapidly evolved (for good or bad): cell phones are essential, wireless is everywhere, social networking has exploded, Napster fell and iTunes rose, iPods have replaced Discmans, etc. The point is this: a lot can happen in 10 years, kid.
I don't really have any advice for you, because I'm not great at advice, and because your 18 is, in many ways, quite different from my 18, and I'm not sure what you're facing. However I can tell you that the world you think is concrete and easily grasped is not, that the reality you accept at this moment will rapidly change and force you to rethink your views on nearly everything, and that the person you are at this moment may not be the person you are even six months from now. The world is fast and relentless and hard and cruel and often enough it doesn't give a shit about you. That does not mean, however, that you should stop giving a shit about the world.
There are practical things I guess I can tell you: don't be stupid about your student loans and fall into the "Oh, I owe $50,000 anyway, I guess I'll just charge these way out of my price range jeans" trap. Don't leave your drink unattended at a party. And I'll stick with a line from that Wear Sunscreen song: "Don't read beauty magazines, they will only make you feel ugly." And perhaps the best thing you can do, I think, is to surround yourself with good people. It might be hard to find them, but they're out there. These are the people who will make you laugh, who will call you on your bullshit, and who will keep you sane in a world that seems to be inching ever closer to total madness.