I knew I wanted to do anesthesia in high school- keep an open mind but ignore the people who think you're crazy for knowing already. (I'd done summer research program that put me in the OR the whole time, and my HS had an affiliation with the local teaching hospital where a few of us did observerships half the day for our jr and sr years.)
I would caution you not to do the BS-MD programs, where you apply NOW to a 6 year combined college and medical school degree. Go to a 4 year college, work hard, and have fun. It will help keep your options open even if you're 100% sure you want to do medicine, plus I think the intellectual and emotional growth you undergo in those 4 years is invaluable. Good luck!
I would agree that undergrad is a great time and you should definitely enjoy those years, esp by traveling abroad and whatnot. However, I have an opinion on combined BS/MD programs as I was initially a part of one at one time. First of all, MANY combined programs are not 6 year programs - there are a number of programs that are 7-8 years in length (3-4 years undergrad, 4 year medical school). Second, along the same lines, MANY are binding - in the sense that if you decide to go to that program, you cannot apply out to other medical schools and must go to that program. The program I was a part of, fortunately, was not binding, so I applied out like everyone else but did not risk forfeiting my spot. I got into a MUCH better medical school, and am happy for that now. Again, along the same lines, I know people in my program who continued with the program, but would have had NO CHANCE of getting into a decent medical school or medical school at all had they not been a part of the program. I think in a lot of ways combined med programs are great things for the people who get accepted, but I've seen too many people go to medical school who probably would not have gotten in had they not been in these combined programs. If you HONESTLY feel there's no way you wouldn't get into medical school, then go to a place for undergrad you would really love. But if you can get into these programs, the guarantee of going to medical school is something that's hard to beat (with the exception of those aforementioned 6 year programs which tend to be pretty intense, and at that point, you can't turn back from medicine b/c your credits most likely will not transfer). So take all advice with a grain of salt and good luck.