Don't take "junk" courses. Some high schools give the same academic credit for a short course in reading science fiction as they do for a half year's study of trigonometry. No selective college takes the same view, nor gives the same weight to a course in American history as to workshop session in "school leadership." And they don't weigh "sci fi" in the same scale as trig.
If you aren't used to answering the kinds or types of questions given on national standardized tests, then, contrary to some opinions, you would do well to practice on your own and be tutored by an expert in just how to deal with multiple choice questions and questions based on a difficult reading selection.
Study a test's design well enough, before taking it, to know whether points are taken off for wrong answers. If not, then guess all you want when you don't know an answer. If points are subtracted, then guessing should only take place when you've been able to eliminate "for sure" at least two of the possible five choices.
Do something a little different each summer. Maybe summer school for enrichment one summer; another year a camping experience; another time a community service project; another, a paying job.
Ask your high school to invite colleges throughout the country to send representatives to your school to meet with those interested. Some colleges will ask local alums to take on that responsibility; others may even send one of the admissions' staff. Don't (repeat don't) sign up to attend a college that you have not seen or a representative of which you've never met. Many high school counselors have never been to more than 50 or so colleges to check them out, and are keeping you from knowing another 2,000. Keep investigating. Visit some colleges yourself if you can.
Be sure you compete in at least one high school sport.
Be sure you make the debate team.
Be sure to be involved in one or more service projects in the community, along with your family, or sponsored through the school.
Study a second language continuously, reading original works in that language; at first children's books and later serious works as well as current newspapers. Many of the large public libraries regularly subscribe to at least one foreign langauge daily newspaper. If you can't get hold of a newspaper in the language you are studying, then ask the school to help the class subscribe to a weekly or monthly news magazine.
Make sure that the school gives you some credit for work experience that you relate to your studies. And if you can't find such employment, ask the school counselor to help.
Participate in sports activities. If you aren't interested in the four or five "letter sports" offered by your school, adn don't want to participate in competitive varsity and junior varsity athletics, ask your school athletics department or principal (or both) to include dance and individual sports activities in the program.