I don't know about Navy, but here's what I can tell you about my experience with Army:
When I did my interview last fall, the Major interviewing me said that they were looking for someone who was a "SAL"- a student, an athlete, and a leader. He also added that they wanted to see if someone would be a good fit for the service.
Keep your grades up, take honors or AP classes, and do well on the SAT and ACT. Taking SAT prep classes will help.
Athletics is pretty self-explanatory. Find a sport you're good at, then excel. They look very highly at varsity athletes, and if possible, become a Captain or Co-Captain of your sports team(s).
Leadership should be both in and out of school. Try to find clubs at your school that interest you and become a club officer or committee head. Also think about community service that you can participate in to show that you give back. Work experience also doesn’t hurt.
Now, you’re only a freshman, so don’t stress out too much. It’s great that you know what you want to do! Right now, just think about building up your resume. I would suggest copying and pasting people’s suggestions into a word document and saving it for later on, if you still think this is the path you want to go.
Good luck!