Some early advice:
The first things have already been said. Keep out of trouble and do well in academics. Take the hardest courses possible. High school is one of the first few tests in life. It is mostly your show. Grades in high school matter and they usually lay a foundation for your life ahead (college options, internships, etc.). Staying out of trouble isn't too bad if you have good decision making skills and possess good character.
Start thinking about what really interests you. USNA loves to see passion on resumes. The "quality of quantity" statement gets tossed around a lot. It looks better to have two or three activities with heavy involvement and leadership roles than every club and activity under the sun with superficial participation. Really dive deep into those passions. For me, they were Boy Scouting and aviation. I earned my Eagle Scout at 16 and have held a multitude of different leadership roles that increased with responsibility each time. I am also working on my Private Pilot's License. If possible, try to seek out leadership roles in the activities you pursue. USNA uses your extra-curricular activities to gauge your leadership potential and your ability to work towards a long term goal (Such as Eagle Scout or Gold Award for Girl Scouts) as well as to get to know a bit about you.
If you are not already an athlete, become one. You do not have to be a varsity athlete your freshman year, but USNA factors in athletics/physical fitness into their evaluation of you. They want to admit applicants who have the physical fitness to make it through Plebe Summer, the Academy, and out in the fleet. Plus, you need to be in good shape to withstand the rigors of flying an F/A-18, EA-18, or an F-35. Club sports are good substitutes, but the Academy prefers varsity athletics. Also, I have heard that USNA has a slight bias towards team sports over individual sports because teamwork is needed to be a MIDN. Not a requirement by any means though. Plenty of XC runners, Track and Field stars, and swimmers at USNA. Do what interests you, not what you think will get you into an Academy. Seek captain or co-captain slots too. Sports leadership is something USNA loves. It shows your ability to adjust on the fly, make quick decisions, and motivate a group of people to work towards a common goal. Lastly, try and get recruited. You have nothing to lose and everything to gain. The worst that could happen is that a coach decides not to recruit you and you are just like any other applicant. If you get recruited, your chances increase.
One lesson I have learned through the application process (going through it a second time) is the power of networking. USNA and the Navy in general has a tightly knit and successful alumni network. Just don't hide your desire to go to USNA. You would be surprised about how many people you meet with some connection to the Navy/USNA. All it takes is for someone to say "Hey, I have a buddy who went to Annapolis." Ask for an email address and introduce yourself That person will very likely know people with rank and those people know people. See it's a snowball effect (in a good way). One of my dad's classmates from college ended up knowing VADM Miller. He was relieved by VADM Carter right before I applied, but nevertheless, you never know what could happen when you say "I want to go to the Naval Academy."
Best of luck! Also, take a look at this link:
http://www.usna.edu/Admissions/Steps-for-Admission/General-Advice-for-Grades-9-12.php