I would agree with biology, and preferably any other advanced science classes you can take. Also, make sure you're taking math classes as well, particularly statistics as science classes use statistics all the time, and it'll be important for you to understand statistics as you're reading scientific papers and journals. I would also see if you can shadow a local doctor, or volunteer in a clinic or hospital in your spare time. Consider getting CPR certified and maintaining that certification. Once you're in college, you should also take EMT classes, as that can set you apart when applying to med schools.