Hi! First off, I would recommend that you make sure this is really what you want to do before you structure your life around it. Almost any medical practice would let you shadow a dermatologist and maybe even work at a practice.

If you decide that you do love dermatology, you should make sure to get a solid grounding in biology, chemistry, and physics. It doesn't matter specifically what you major in, so long as you have a very good science background. During your junior year, you'll take the Medical College Admission Test (MCAT) and looking at medical schools that have good dermatology departments (your college career office can help you find these), and apply to those during your senior year. Once you choose a medical school, you'll spend three years studying medicine and getting ready for your United States Medical Licensing Examinations.

After that, you'll choose a dermatology residency program, that will take an additional two years and give you practical training in the specialty and take the American Board of Dermatology Examinations. If you pass these exams, you'll be a licensed dermatologist and can then either begin practicing, or complete a fellowship that will give you more experience in a specific area of dermatology.

hcook3 gave a pretty detailed answer but I can only add that dermatology is really competitive. It's one of the hardest specialties to "match" into. The residency "match" is what places you into a dermatology residency program and only 30 - 40% of people who apply (keep in mind, that only the people with really high USMLE scores even apply to do dermatology) get placed into a program.

That being said, dermatologists make a lot of money.