Hi!
First of all, I think it's
awesome that you've started looking for your future this young. I wish I had myself. Kudos to you.
For grades: as high as you possibly can. Aim for the 4.0. The higher it is, the better your class rank will be, and the more options you will have when the time comes to apply to colleges and for scholarships. That being said, I do not believe there is any
minimum gpa. If I remember correctly, nearly all of those attending West Point graduated in the top 25% of their class. I have a 3.496, 19th %, and a LOA, but my LOA is only because I got a 34 on my ACT. However, I am disqualified from many scholarships that I could use towards a civilian university because of my low GPA. And if I hadn't slacked in 9th and 10th grades, I could easily have a 3.8 or higher. So, aim for the 4.0.
For classes: focus on math and sciences. You want to take at least Pre-Calculus by your senior year, and Calculus if you can. Take at least one year of Biology, Chemistry, and Physics; two years of each if you can. Take a foreign language, and stick with it throughout high school. Take four years of English classes, focusing on composition and reading comprehension. Another course that is often recommended is economics, so take that if you can as well.
Good luck throughout high school.