As your questions are fairly vague, I'll tailor my response equally vague. Go to college and get a degree in something you enjoy. Most agencies will hire an applicant without a degree, but if you want to advance, a degree will help you achieve that.
Stay out of trouble, you're coming out of your "young and naive" days, and will have to explain slip-ups with the law, to your perspective employer during the interview process.
You'll have to work the road for at least a couple years if not far longer, so if you don't want to work in patrol, then you may want to consider a different route to attain your goal of working in investigations.
The best advice I can give you is to figure out where you want to work (i.e. FBI, local, wherever) and ask them what they look for in a potential hire, maybe even see if you can volunteer with that particular agency.
Good luck!
Astronomical avatar of the week: The Hourglass Nebula: This formation of gasses was created when a dying star gently ejected its outer layers forming a glowing region of ionized gas as seen in the photo. The remaining core is known as a white dwarf, seen in the center. Our sun will leave a similar corpse behind after expending its nuclear fuel. This nebula is located in the constellation Musca, the fly, visible only in the southern hemisphere.