How does one become a doctor for the military?

I while ago I came to the realization that if I could do anything with my life, I would really like to be able to help other people. For me, I think this means being a surgeon. I haven't yet decided what I want to specialize in, but I've considered being a neurosurgeon or a cardiovascular surgeon. I would really like to know all my options and I've been considering working in the military. I would really appreciate if someone could take the time to tell me what I would need to know in order to get there as a doctor. Thanks!