Sit down and think about it. Do not just do something because it is expected of you or because your friends or girlfriend is doing it.
That time of age for me was wicked fun but at the same time I had no idea what I was going to do and I never actually sat down to think about it. I just went with the flow so to speak. And when the time came I panicked and ended up wasting 40k on college that I dropped out of because I was shit bored with it and still had no idea what I wanted. But the experience was fun.
I could rant all night about that but all in all just sit and think. DO not be afraid to talk it over with friends and family. It helped me when I finally decided college was not right for me at the time, and other decisions of major epicness.