I remember being in high school, earning top grades, taking advance classes (honors and AP), extracurricular activities, and also the same eagerness to want to become a doctor. It's a long road ahead, but in hindsight, it went by pretty quickly
Anyway, the most important thing you can do while in high school is make sure your study habits are part of your daily routine. Good study habit is needed to succeed in college and also the first two years of medical schools. Makesure that your study habits become part of your daily routine.
The biggest problem that people run into when starting college is the sudden rush of freedom - professors do not care if you don't do your homework, or don't show up to class (especially large lecture halls). No one will yell at you to go to bed at 3AM. In college, you'll be around people your age, who are partying, slacking off, drinking, or doing other "activities". For some, these distractions can have serious consequences on their grades. But I'm not saying you can't have fun or enjoy yourself while in college - this is where good study habits and coping skills will come in handy - you need to find the right balance between having fun and being studious.
As far as courses go, make sure you take Chemistry, Physics, and Biology. A strong foundation in these basic classes will help you when you take these Intro courses in college. Don't forget English too - there is a verbal section on the MCAT (where traditionally students have the hardest time)
Be yourself - be well-rounded, enjoy life. Don't rush things - sometimes I wish was back in high school (well, the nostalgic part at least) instead of doing 30 hr calls q4 and a 2-day weekend is a rare luxary (and termed "golden weekend")