Pick a major that interests you so you don't mind devoting a majority of your hours to studying. You will need to get good grades in college in order to apply for medical school. At the medical school I attended, the average GPA is reported to be 3.85, so even one or two B's can hurt your chances of acceptance.
Aside from this, any major is acceptable as long as you complete the prerequisite courses. I chose to major in biochemistry because there was overlap with the premed requirements and I wanted to complete my degree in 3 years.
Medical school takes 4 years to complete.
After medical school dermatologists complete a 4-year residency for additional training.