Hi, and welcome to JpnRef.
Before being recommended, you would have to pass the primary screening conducted by your embassy/consulate (which includes document screening, several academic exams, a health examination as well as an interview). Even if you were to be recommended by your embassy/consulate you would have no guarantee of being selected as your application would have to be in the top 100-ish when compared to those of all around the world. To put it differently, as there exist no quota of how many scholars each country can send (in theory at least), it isn't enough to be one of the best in "just" your own country, but you would have to be one of the best 100 high school students globally. If you feel that becoming one of the top high school students in the world (admittedly somewhat over-simplified, but you get the picture) is too much of a challenge, you could go for the Specialized Training College one, the College of Technology one, the Japanese Studies one, or the Research one (which are all objectively "easier" to obtain, with the Research one being the easiest (still very difficult though) and the Undergraduate one being the most difficult). Just for a reminder, the scholarship is in no way any kind of charity for manga/anime/Japanese culture-loving people, but a scholarship meant for aspiring academics. If you are still up to the challenge, I would suggest starting off by becoming one of the top high school mathematicians (with a grain of salt) in the USA, as the math exam weighs a lot compared to the other. I would also suggest studying chemistry and biology/physics if you are interested in studying natural sciences. Somewhat discouraging news might be that the USA hasn't sent that many students for the Undergraduate program (compared to its population of 300 million people).
Good luck and aim for the stars!
Regards,
Sarah