Can I join the US air force as an air traffic controller, and then use my ATC skills from my service to become a civilian ATCO?
Yes, military air traffic controllers often transition to the civilian system after their military retirement. But if your end goal is the commercial world it might not be for you. The military want people fully committed to the job - committed enough to serve in a war zone where people would love to drop a bomb on your head. It is not something you can do for just a year or two and then move on, the minimum service commitment is, I think, 8 years. If you are motivated enough to accept all that then you can sign up after high school.
In the US the civilian ATC is run by the Federal Aviation Administration. Their requirements are on their website, and it looks like you'll need a degree or three years of suitable work experience. Once you meet those requirements you just wait for an advertisement and apply.