Good Luck pursuing your dream!!!! Speaking as a parent, I would also advise you to keep busy during the summers by doing some sort of volunteer work in your community-something you are interested in: volunteer at the animal shelter, coach 5 year olds how to play lacrosse, whatever.
Look for leadership opportunities in your school, neighborhood, and community. If you see a need and no group exists, consider starting one. When you are 16, get a part-time job while still earning top grades in all your classes.
If your high school offers the " start taking college courses as a junior" type of program, apply. You will be taking college courses in combination with your high school classes. Ask your high school guidance counselor for details on the local colleges that offer this type of program.. Science, math, more science, more math, and engineering type classes in the "High school-college" program.
Apply to NASS if you decide on USNA, or the other SA summer programs if you decide USNA is not what you want. The summer programs give you an idea of what that particular academy is like. When DS was at NASS, a couple of years ago, the detailers were watching them closely to see who would be a good fit at USNA as a mid. DS said that by lunchtime of the second day, half of his squad had already decided they were no longer interested in USNA. DS , on the other hand, loved every minute of NASS. He is finishing plebe year right now.
In a nutshell, the academies are looking for candidates who have a LOT on their plate and handle all of it very well.