I'm 18 and I have to say I'm scared as crap. I'm already a senior in high school and I'm about to graduate! I'm not sure what I will do with myself in the real world, but I'm sure I will be ready for it. Enjoy your age and once you hit the big 18, don't make stupid decisions. Don't get a credit card or spend money all the time, you will regret it later.
its good that you're worried about it, but i'm 21 and i hate being an "adult" it sucks, so my advice for you is: have fun, be a kid, enjoy it as much as possible. my childhood sucked so it was ruined for me, i regret it every day. i really wished i would have tried to chill out and enjoy life more as a kid. i got into drugs at a very young age, drinking, smoking, pills. you name it i was doing it (except crack and stuff) i grew up way too fast. when i was 12 i was trying to be so mature and bad ass all the time. i really wish i would have chilled out and just tried to be a kid. honestly my best advice is to just be a kid. don't think about the future.