PathOne is right... one step at a time. Worry about getting into med school first, and then worry about building up your resume for a derm residency.
One thing that might help is, if your undergrad has a medical campus, get in contact with the derm department there (if there is one), and perhaps do some research with them or shadow some dermatologists around. If you end up actually going to there for medical school, you could continue your research with them, and then when it comes time for residency, assuming you have all other factors in order, you might be able to match there. Either way, building some contacts in the derm field might help.