Take difficult classes - AP if your school offers them, Honors if not. Some would say "take the hardest classes that you can still get good grades in," but I would submit that if you can't hack at least a B+ in a High School AP class, then you're going to find the Boat School academics pretty damn tough going.
USNA looks for the "whole person". That means someone who's not a jock, nerd, or member-of-every-damn-club joiner, but a mix of all three. They want someone who did well in classes while also playing a sport and doing a few extra-curricular things, plus maybe hold down a part-time job. In other words, someone who would be able to handle tough classes and manage all the extra demands when they're a Mid. And they'd rather have someone who did all those things pretty well, rather than someone who excelled in just one area.
(note: credits don't transfer, but USNA will take AP exam scores as validating some classes. I got a 5 on AP English, which validated me out of a semester of Plebe English)
You're doing the right thing by starting the process as a freshman. It's not at all early. Keep your grades up and get involved in whatever interests you and you're willing to stick with (Admissions knows what a "joiner" looks like on an application, being a member of every single club the school offers won't impress them). Play a sport - you don't have to be team captain, just show that you have some athletic ability. If you are so physically hopeless that you can't get on any team...USNA may not be for you.
Strongly recommend doing Summer Seminar when you're a Junior. It's a great full-immersion program and gives you a good taste of Middie life. And do the overnight visit as well - it's like a pre-frosh program where you drag with a Plebe or Youngster overnight. My advice is, drag with a Plebe and do it during the Dark Ages (Jan-March).
Talk to your Blue and Gold officer with any questions. They're not so much recruiters as sources of wisdom and reality-checkers. The Boat School gets enough applicants that they don't have to talk anyone into it who isn't interested, or who they feel isn't right for the program.
Good luck!
Fester USNA '99