I'm sure this'll get moved there, but there's a pretty decent write-up about it on the main page.
There are three main routes:
1. Through the military; specifically, by being a military air traffic controller. A minimum of 52 weeks controlling is necessary to apply through this path, although the shortest commitment is four years (Army and Navy, I believe). This also puts you on top of the hiring queue, since you'll have actual controlling experience -- there's no substitute for that. Not schooling, not piloting, nothing.
2. Through CTI (collegiate training initiative). It's a college program, put together by individual colleges and approved by the FAA, that allows to apply via a specific application opening that is generally open all year long (but considered on a quarterly basis). Prices and lengths vary, from expensive four-year schools to relatively cheap community colleges. If you already have a bachelor's, you can complete some programs in as little as one calendar year. You've got more of an edge than the last category, but since no college course can compete with actual controlling experience, you may have to apply a few times before getting in.
3. Off the street (OTS). From time to time, the FAA has create openings for the general public. They're sporadic and depend on need... and at the moment, there's no longer a need, at least for a year or two. Two years ago, a series of 8 announcements for OTS applicants went up over the course of around ten months, but a combination of overhiring, the crashing economy, and budget issues made it so OTS hiring has been discontinued for the time being. Its never been a reliable way to get in -- kind of a gamble, really.
