You need to fulfill a standard set of prerequisites for med school - math, physics, bio, chemistry, etc. It's easiest to major in one of these subjects, but it doesn't really matter - you can get in just as well if you major in something else but take these classes.

Depends entirely upon what you want to do. You could join the services (Army, Navy, etc.) and they will pay for much of you advanced education but you wil likely end up a GP. If you want to work in a research lab such as the Center for Disease Control, then micro-biology or communicable diseases should be your focus.

Then again, where do you want to live? Walter Reed Hospital area? Bethesda, MD?

Over and above a desire to work for the government, what are you really interest in? Whether you want to cure cancer or mend broken bones, do what makes you happy. There will be an opening for you somewhere. No sense specializing in something you really don't care about. If you do what really interests you, it will not seem like work everyday.