Look at what
@ca2midwestmom said above. Many do apply to all the service academies. They have the noble intention to serve our country, through the military, any way they can. Then there are others that are more selective in what branch they want to pursue and focus their attention there.
My DS is Navy all the way and I will share with you his rational. My DS would love to be a military aviator, but his uncorrected eyesight is not good enough. He realizes that once he is in a Navy program (USNA or NROTC program) it MAY be possible to have corrective eye surgery that would enable him to fly. So service first, pilot second. If you want to fly, and you want to fly fixed wing aircraft, Air Force would be the way to go, but with DS questionable pilot medical qualifications he was not happy with the other options the Air Force has to offer. On the other hand DS loves the water, he sails, rides jet skis, scuba dives, and spends lots of time during the summer in the ocean and on the water. Being on a surface ship and being paid to be there would be a dream. So for my DS a Navy program gives him options that are more attractive, and he will have time to decide. I am not sure why he hasn't pursued an Army option. He is a boy scout, he loves the outdoors through camping, hiking and backpacking. But he probably just can't see himself lugging a 60 pound pack and sleeping in the mud. I would have thought Coast Guard would have been a good option because of his love of the water, but for some reason Navy just "feels" right. I don't know if any of this helps, but you have plenty of time to figure it out.
Most importantly, go to college. If you agree with that plan it doesn't really matter if it is a SA or a civilian college.
Get good grades in the most challenging courses available.
Do well on the SAT/ACT
Be involved in EAs and become an officer or better, president of a few of those clubs
Do community service
For the SAs they would like to see a commitment to physical fitness and leadership through sports. So participate in HS athletics, get a varsity letter and be captain of the team (all of which will help a civilian college application)
Best of luck