Once in college, do the same, but start shadowing physicians and volunteering as well. This will provide a better understanding of the day-to-day operations. Helps with letters of recommendation as well.
Deciding on a medical specialty (Dermatology, Pediatrics, Emergency Med, etc.) doesn't happen until the end of medical school, so until then, continue to explore different areas of interest and keep your grades (especially your science grades) up!
The subject that you'll need in high school for the road of becoming a Dermatologist is to take anatomy, physiology and health science. Also pharmaceutical technician classes because you'll be prescribing some medications to your patients , and needs to know the correct ones. Basically all the medical classes you can take, I will suggest you to take them.
Hopefully that helps.