First off, it's good to see you're looking towards your future in high school! Many states require police officers to be 21 or 18 to 20 if you served in the military.
There's many things you can do in preparation. Most large and medium sized departments require an associate degree or competition of their Academy and college, completion of their academies are always required. So, here are my three thoughts, wisdom or ideas :
Serve a tour of duty in the military, any branch, preferably military police.
Go to college, try to get on with a police department as a volunteer, ride along or pioneer, different places call it different titles.
Go to college and work as a security guard, firearms instructor or the like. Once completed, apply for the local department's Academy.
Finally, you're doing well thinking about your future! Stay determined, don't get caught up with stupid people and stupid things! Become a sponge, that means learn anything and everything about policing. Avoid being taken in by negative stereotypes of police, there are good and bad people in every organization, good news is there are usually more good!
Wish you the best of luck with your future endeavors and you're using a great resource of information from many adults that have years of experience!