You really should read their website.
Being someone who has gotten to Phase II of the process, you'll need at least a college degree. You'll need some kind of speciality (Cisco cert, CPA, etc) or a masters + x years of experience, or have background in the armed forces. You'll take a written test in Phase I. If you pass that, you'll be in the pool for Phase II selection. The pool for selection just to be picked to be interviewed for Phase II is pretty hard...so unless you have that specialty, you're pretty much going to be overlooked.
Phase II testing is hours upon hours of panel interviews, psychological tests, etc. Assuming you get chosen, you get sent to the academy for XX weeks, then assigned to a city of their choosing.
This was the process two years ago...I don't know if it's still 100% this way but that's what you're looking at.