Take Chem, Bio, Physics and AP Bio or AP Chem.
Do well in your other classes.
When you can, start to volunteer at a hospital or nursing home to get an idea of what it is like working with patients.
Check out these tips: http://talk.collegeconfidential.com/college-life/1920853-college-is-a-step-up-from-hs-16-tips-on-doing-well-in-college.html