If I had to give advice to any high schooler (and I sincerely wish someone had given me some - I'd have saved myself 5-6 years of sleepwalking through life).
Have a plan.
That is really all you need. I guess I can flesh it out more though

.
Life doesn't just happen to you. You can actively influence the path your life takes. Are you going to be the next Bill Gates? No, probably not. But if you started off anywhere other than a ghetto in the city somewhere, you've already had everything except the most crucial ingredient handed to you on a silver platter.
When I graduated from high school (which was sadly... last century [lol]) I did not have a plan. I wanted to enlist in the Marines, my parents wanted me to go to college and offered to pay for half. Since I didn't feel strongly either way, I took them up on the college offer, and attended a nearby state school (UMBC - it is actually a good school). The main problem with that is I didn't have a reason to be at college. It really wasn't part of my plan (to be honest though, since I didn't really HAVE a plan...).
So I bumblefarted around for two years, taking general study classes (note: if you are going to do that.. do it at a community college.. that is what they are there for). I'd go back and forth, declaring different majors every semester. Finally I decided that since I liked history, and it was easy, I'd do that.
Fast forward to 2000. I hadn't done any summer internships, I hadn't even spent the last semester looking for a job (I was working at a company that did technical support and testing for video games, which I really enjoyed, so I didn't bother). Got fired from that job (along with about half the company, they lost their two biggest customers) in December of 2000, the same month I graduated from college with a handy History B.A.
Now that that point I still could've turned things around. I could've tried for a military commission (especially after Sept. 11, although I was, and still am, a lardass - working on changing that now). I could have tried any number of things. Long story short, I didn't. I sleepwalked through life until around age 25, at which point I realized that even though I wasn't alone in having a dead end job and living with my parents, that didn't make my life any less of a collossal failure.
But the most important part here? I didn't give up. I accepted the fact that I had, up until that point, blew it - but I didn't write myself off. I decided to go back to school. I have friends that STILL haven't (I believe they've given up, and it wouldn't surprise me a bit to have another one kill himself). I got a math degree - which I am paying for now. But that was an investment. One that will pay off. I have a great job with the Army (as a civilian). Once I get in good enough shape to pass the physical, I plan on joining the Guard in my area.
So now that you know my life story (LOL) - I hope that you can take something away from it. Learn from the mistakes I made.
The military is NOT a bad thing. I think most kids would benefit from serving for 2-4 years. It gives you a taste of what the real world is like, it offers you a possible career, but more importantly it gives you time to think about what you REALLY want out of your life. If you don't want to do the military, but you don't really know 'what you want to be when you grow up', then do the community college (or a four year school but get VERY involved with extracurricular activities - i.e. internships, etc.. not the yearbook club).
Anyway, if you want to talk in private, feel free to contact me via PM. Or post back here and I'll email you. If you want to talk in public, just respond here. I'm absolutely willing to talk about the fits and starts I experienced in my life to this point if it helps someone else avoid them.