Will these exam results appear on your official high school transcript? Will you need financial aid?
It is VERY difficult to get into a top American university. It can also be VERY expensive. To get into an elite school with financial aid you need to be among the top students in your country. It also helps if you have some highly prestigious honors, such as international olympiads, or other significant accomplishments.
To prepare yourself now, start reading about the American university system and what it takes to get into an American university.
https://educationusa.state.gov/
This site will also tell you if there is an advising center in your country.
Read throughout this website, including the Financial Aid, College Selection and Parents forums. There is a great deal of information there. Start studying for the SAT and SATII exams. Very, very high scores are necessary to get into the top schools. Also speak to your high school teachers or your principal about writing you letters of recommendation.