Answer:
Stay physically fit. Study science and mathematics. Keep good grades. Learn about the Solar System. Take flying lessons.
Answer
If you are not planning on getting a B.S. (or masters or doctorate), you better start making plans to do just that. Ain't nobody with a high school diploma and nothing else getting a ride on the big candle. Consider enlisting in the military after school and getting into a flight program to be a pilot.
Bottom line is that you'll have to do very well in school, matriculate in a "hard science" that has some relevance to space, and learn to fly somehow. And establish and maintain a superb state of health along the way, both physical and mental.