Krav and Brazilian Judo are the two I would look into. So your 18 and you want to be a cop. I hate to burst your bubble but if your a white male with no military background, you will have an UPHILL battle. In 3 years you are going to have a lot of vets from Iraq looking for jobs. Even if you Ace the entrance test, you will have a boatload of vets that will get extra points for being in the service.
Things that will help you get in.
1) Language skills..If you can speak another language that is a definate plus
2) Clean, Clean , Clean record. The teen years are the hardest times to stay on the straight and narrow, but try and don't associate with anyone involved in drugs or other criminal activity.
3) MILITARY. Tough decision since we are at war, but like someone else said it too, if you can get MP duty it will help a lot. There are supposely 2 year enlistment options in the military now, you could check that out.
4) Get into and stay in shape...real simple there are standards in running, weight lifting, etc for most departments, find out what they are and get fit enough to pass, and stay that way.
5) Some departments REQUIRE that you have a college degree. Make sure your the department your interested in does or does not require a degree. If it does not, don't waste you time going to college unless you are already interested in going. Most police department will actually pay for classes once your in. A CRIMINAL JUSTICE Degree does not give you any special treatment in Law enforcement over any other Degree (trust me I know this from experience). If your interested in a Fed position and they require a college Degree, Minor in CJ and major in something useful. Also a bonus would be to do ROTC while in college, so you get a degree and military in one shot..but it will be a long haul.
Hope this helps and good luck.