NASA - National Aeronautics and Space Administration

Advice from Former Ohio Astronaut Don Thomas to High School Students

The main thing I tell the kids is to study hard, do their best and stay in school. I also tell them not to ever give up on their dream if it is something that they really want to do in life. Rejection and failure is likely, but persistence can pay off in the end. (It did for me.) The range of career paths is quite broad (military, engineering, physical sciences, etc.), and I would recommend you try to get in as good a school as you can.
I would also track who gets interviewed and selected in the new astronaut classes and see what their backgrounds are. It is usually a pretty good clue as to what type of individuals they are looking for and what traits may increase your chances of selection. Most folks get selected in their early to mid 30's, so you've got time to work on your background.

If you're interested in the civilian route, I would encourage you to stay in school for a Ph.D. It's a lot of time and effort, but it would always serve you well and will significantly increase your chances. (Take a look at the numbers selected for civilians with and without Ph.D.s... it's a pretty convincing argument.)

I hope some of this helps. There's no magic formula, but I think there is a lot of good advice to do what you can to optimize your chances of getting selected.

More Information:
>  Astronaut Selection Web Site →
>   Don Thomas Biography →
>   Ohio Astronauts