Volunteering is a great start. The other thing you'll need to add is shadowing-essentially follow a doctor around so you can see what it's really like. If you're able to spend the next three years doing those things on a regular basis, AND maintain a perfect or near-perfect high school GPA, AND after all of this you still want to be a doctor, then a few colleges offer joint BS/MD programs. Typically, admission is very competitive, but if admitted, you complete only two years of undergraduate coursework and immediately enter medical school. Usually you only have to meet minimum requirements during your undergraduate years. These programs are only for very hard workers who've had a lot of experience with hospitals/doctors in high school, and the very large disadvantage is that in college, you don't get to choose any of your courses. You'll never be able to study music, or literature, or politics-it'll be all science all the time, and on graduation day, you risk being a medical idiot savant with no friends, no social skills, and no experience with life outside of medicine. (That last part is just my opinion.)
Good luck with the volunteering!
