When I graduated from high school and went to college, my brother and I took full advantage of the freedom, and cut class, partied, etc. We were fortunate enough to have parents that paid our tuition, and we wasted their money for the first two years. Our grades got worse and worse each semester, and though our GPAs were never low enough to get us on academic probation, they were pretty pathetic. One day, after our first couple years, our dad sat us down and told us if we weren't going to take it seriously, then drop out and go back when we will. From that point on, we took things seriously, and we both finished with fairly strong GPAs, but it took those final couple of years to get out of the hole we had created for ourselves. Fortunately, I got my GPA high enough that I can go back and get my master's without having to crush the GMAT.
So, my first piece of advice: Take school seriously. Education is important. I know when you're young, the whole "C's get degrees" way of thinking is cool to you, and all you want to do is party and hang out with your friends, but you really need to make it a priority. I'm 27 now, and a lot of my friends who took school seriously are finishing med school now or are on paths to much better careers than me. Some went right into the master's program while I was off fucking around with my friends. At the time, you look at these guys and you criticize them for doing nothing but going to school all the time, but I promise you will reach a point where you wish you would have done the same. If you can go right into a master's program, do it. Don't waste any time. I have wasted several years now that I can't get back, and I have very little to show for them. Set yourself apart from the rest, and continue to get an education.
My second piece of advice: Do something with your life. Make money. To quote the movie Boiler Room, "Anybody who tells you money is the root of all evil doesn't fucking have any." Honestly, I went through the whole phase where I thought making a lot of money was dumb and had the whole "I just want to be happy mannnn fuck money" phase, but you know what? being broke sucks dick. I know a bunch of hipster kids who think $10/hr. is all they need to be happy, but it isn't. One of my best friends got himself in a situation where him and his girlfriend have a baby now and have to rent a house, and they are dead broke. Guess what? they aren't happy. He's miserable and depressed, and he complains about it constantly. I know a lot of people living paycheck to paycheck, and it's not fun. There is nothing charming about being broke, so get off that whole idea as fast as you can. Go make something of yourself, you'll be happy you did in the long run.
Don't be one of those guys that says they can't afford to go to college either, that's bullshit. There are so many loans available, that anyone can go to college if they want to. If you say you can't afford it, that means you're too lazy to look for help. Start at a community college if you have to, get your undergrad work done and get good grades, and transfer to a good university. From there, you can do anything you want. A typical response to this will be "Bill Gates didn't finish college...", but go down the Forbes 400 richest Americans list (http://www.forbes.com/wealth/forbes-400), look up each guy, and see how many didn't go to college. Chances are, you will never be the guy who didn't go and end up on that list. You probably won't ever be the guy who did go and end up on that list either, but you'll definitely be improving your chances.
To sum things up, take school seriously. There is plenty of time to party and hang out with your friends, but make school your top priority. There is nothing uncool about doing well in school, and you'll be glad you did in the end. Take full advantage of everything your school has to offer, join clubs, volunteer, go to games, etc. I avoided all that like the plague and I regret it. Go out and make something of yourself, and don't be embarrassed to earn some money. I know too many people who are broke and miserable to ever believe that not having any will make you happy.
I'm sure this thread will be trolled all to hell, but hopefully some of you can learn from my mistakes and get started on a better path.