Choose a field of study that you like and are good at. NASA is always looking for people with backgrounds in science or engineering - not only math and physics, but also biology and other related fields. The minimum educational requirement is a bachelor's degree in engineering, science, or math, with at least 3 years of related experience. Advanced degrees are recommended. Learn some extra languages With the building of the International Space Station underway, space is becoming a more and more international place.
Useful ones are probably Russian, Japanese, and maybe Chinese, French, and other European languages.
Be in good physical shape NASA wants astronaut candidates who are in good physical health to put up with the strenuous work required in space. So work on being healthy - play sports, work out, and have fun. If you become an astronaut, you'll have to spend days and maybe months cooped up in a small space with a few other people. So interpersonal skills are very important. Work on getting along with others, teamwork, cooperation, and communication.