[url]http://www.fbi.gov/employment/agent.htm[/url]
That link will tell you most of what you need.
AS for what you can do to start preparing yuorself... The NG is a good start. Also, if you aren't already a runner, start running on a regular basis. (I say this only cause I could never get myself into the habit of running enough to meet the requirements without feeling like I was going to die. Not that the requirements are that difficult (1.5 miles in ~12 min, don't rememnber the exact time). I just hate running)
For languages, if you ask them, they will tell you which ones are in demand, but that is always changing, so what't in demand now likely won't be by the time you apply, so don't let that influence what language you learn. Spanish, Russian, and now, Afghan dialects, among others, would proably all be good language choices, even if you don't apply under the language option.
As for what classes to take: Obvioulsy law or accounting if you choose one of those options. Anything that interests you and could be applicable can't hurt (pych, sociology, law enforcement, forensics). Keep in mind that it's unlikely you be selected right out of college, so plan on having a job or two between college and the Bureau, preferably one in accounting, law, law enforement, or something else that will build applicable experience when yuo do apply with the FBI.
Hope that helps. Good luck.