Here is a rundown from Stuckmic.com about controller positions and how to apply.
http://www.stuckmic.com/air-traffic-control.htmlIn the "old days" lots of controllers had military experience, but the percentages have lowered slowly over time. (A decent percentage of controllers came in "off the street", as it was called, with no special ATC experience at all.) In the last decade or so, several colleges have been in the business of providing ATC training programs that lead to a degree, but those graduates still have to apply to the FAA and make it through the training program.