go to CIA.gov or FBI.gov and look under their career pages.
Can even do the ecret service.org
CIA CAREER SITE ----- https://www.cia.gov/careers/index.html
FBI CAREER SITE ----- https://www.fbijobs.gov/index.asp
SECRET SERVICE SITE ----- http://www.secretservice.gov/join/index.shtml
Check with your local police station about joining :p I'm in high school still and plan on going SWAT Officer after I'm done in the army. Also consider the National Guard as an option, you can join and go to basic the summer before your senior year, you come back and do school, then go back and do AIT, after that you get sent to your unit with 2 years already fulfilled from you contract! (contracts are 6-8 years, and the last two years of each are basically you being done with the army but still able to be called back if they need you) The national Guard is the only military branch that is funded by the Federal AND State government. 2 or 3 drills ago they said the tuition program is temporarily suspended, but since we are in the National Guard the State tuition plan still has us covered. It's something to consider, don't listen to that "national guard is for wussies" talk. One of the cadre at my RSP was a pro MMA fighter (Saw a video of his fights where he just DESTROYED the opponent) and combatives instructor. He says the national guard is better than the marines (he's been in both).
Police service won't really hep much if you don't have the required schooling. And SWAT is something that takes quite a few years to get on (and that's if a slot is available/open), and it's not necessarily "lower" than FBI, CIA, or other jobs.
Check each link, check the requirements for each job you want within whatever agency. Set your college courses based on that job you pick.