Be very active in extracurricular activities. Volunteer in community service on the weekends, be in at least 2-3 clubs, and maybe even play a sport or instrument!
For your classes, I would take Honors (or whatever the highest level in your school is) for each subject.
Whatever Science you prefer, I would take a base course in it (for example Regents classes in NY) and then take AP in sophomore year. Also, take the SAT Subject Tests in whichever Science you decide to take