Def start by way of the AF. Prior military gives you a foot in the door. Not to mention you can try to get positions working with the other agencies while you are in the AF. When I was in the Army I had a few friends that got white house jobs. You have to apply for the positions and then obtain a TS clearance. If you have 4 years of college you can go into service as a commissioned officer. Also don't forget, you can get stuck behind a desk in any of the agencies including any military branch.
Oh and every Officer has to spend time behind a desk.