1. You are a child. I know, you're closing in on adulthood. But I'll tell you this now: there is not some instant maturity reached upon the transition between seventeen and eighteen. You're still a kid, and that's something awesome. Go do stupid things while you can. Hang out with friends. Take a road trip. Go bungee jumping. Eat on your parents' dime and date high schoolers. Don't think too hard about college until you're actually IN college.
2. NOTHING is set in stone. When I graduated high school, I traipsed on over to China and played my trumpet through four provinces. I was dead-set on entering the Peace Corps and really didn't even plan on college. When I entered college, I was going to major in applied linguistics and STILL join the Peace Corps. Obviously, that's not how things worked out.
I'd say that there's a minimum 70% chance you will change your focus from medicine to something else. You would be doing yourself a disservice by not contemplating other fields. Who's to say you wouldn't be happy as an astronaut or a stockbroker? Maybe your life will be complete with a position as a restaurant owner. Who knows, you may become a PA or a nurse or something. Sure, it's great to have distinct goals. But let yourself have time to make them!
3. Don't follow a set course to the letter. I planned to major in linguistics when I entered university. On my brother's advice, I took other classes - Japanese, anthropology, computer science, creative writing. I'm now currently working towards a double major in Japanese and Anthropology. Take classes in things you wouldn't have considered otherwise. Take psychology or horticulture. Maybe gender studies. Learn a language or take a class in political science. It might end up the best thing you ever did.
4. Take advantage of opportunities. Go wild. I never would've thought to be a doctor until I had to wait a year to give blood, and if I couldn't give blood, I figured I might as well give time. My volunteer experience with the Red Cross opened the door to health care for me, and subsequent exposure to the field is what made me decide to go into medicine. I also accepted an offer to study abroad (best experience of my life) and have attended conferences all around. Accepting invitations and taking advantage of opportunities presented to you will change your life, whether in small (using your own thermos instead of disposable cups at cafes) or big (pursuing a career you never even imagined) ways.