There is nothing you can do to apply to USNA now. You can get information about USNA from the website and/or by attending local service academy events in your area. Members of Congress often hold open houses, as do various other groups. Information is sometimes posted on this site; you can also check your MOC websites or contact your MOC's SA coordinator. In January 2018, applications for USNA's Summer Seminar will open for rising seniors. You may want to apply. There is information about applying to NASS on the usna.edu website.
You might also contact your local Blue & Gold Officer (if known), or if not known, your Area Coordinator (from USNA website), and introduce yourself. He/she might have some good insight into what you should be doing now to improve your application. This is a good time to be thinking about activities and leadership opportunities. As a BGO, I like the early contact and think it shows interest , motivation, and a level of maturity.