You are to be commended for your resourcefulness - I see you have already looked at answers to similar questions.
Because of your age, I recommend you concentrate on making it through the teenage years with your sense of ethics, integrity, and lawfulness intact. This really is not easy to do! It is important to make the "right" decisions, and not necessarily the "popular" ones. Perhaps you heard of the recent situation where a girl pushed her friend off of a high bridge? She was not thinking of the potential consequences, and has now seriously hurt her opportunities for the foreseeable future. Teens today have to be able to navigate social media, without engaging in cyber-bullying or posting inappropriate photos. It's not easy! Beyond that, are "friends" who tease you because you won't engage in petty crimes, like TP'ing someone's yard. Even the smallest of crimes can have consequences. This does not mean that if you make one little mistake you will never get to be a police officer. But, it could mean that you have to start in a less desirable department rather than the one you want.
You also want to be known as a person who keeps their word. If you promise to type a paper for a group project, make sure to do it. Be dependable. Have good relations with your neighbors and teachers. Be friendly, polite, and helpful. Many police departments will talk to these people when they do your background check!
Okay, enough about being a good girl. Next is physical fitness. I'm not a fitness expert. But, there are basically 3 types of exercises: cardio, stretching, and strength training. Discuss these with a PE teacher or coach. All 3 are important. I'm not sure if they still use it, but look at the Cooper Fitness Standards for police officers. As you transition from youth to adulthood, fitness won't come quite so easily. This is why you want to start now to form good fitness habits!
There are various qualities that are looked for in police officers. Ability to stay calm under pressure, Make quick decisions, and keep going in the face of adversity, are some that come to mind. Teamwork too. I'd look at getting into sports and/or the police Explorer league, if one is available.
I really enjoyed my time on the department. If there is anything else I can do to help you out, please don't hesitate to ask! I wish you the best!