There are programs that are available to you while you are in high school and if you are able to get your parents permission. The program is called the Police Explorers. There you are required to do PT, ware a uniform, and participate in various activities. Sometimes they bring in various units like K9, SWAT, Air Units if your area has them, Detectives to talk to your unit. If you are 18 you might be able to do ridealongs but it all depends on your city/county. You must be at least 21 to join due to the fact that you need to carry a gun legally. Once you become 21, you apply, pass a background check, pass a psych exam. Once you pass all that, you go to the academy and do more PT, and class work where you learn about law, driving, and other things. Then you need to pass a probation period ( known as your Rookie year). Once all that is said and done, and a spot comes open, you can apply. Then the process starts over. You get interviewed, then depending on the department might need to do quals to see how accurate you can shoot. Then you go to school a are evaluated on everything you do. A plus that would be a benefit to you and your department would be to get your EMT, or Paramedic certification. A lot of SWAT units have medics on the team. It might help you get in a bit faster not by much but might help.
Good luck hope it helps,