Ivy League schools like well-rounded students.
You should be in the top 10% of your class (or top 1% or 2% for the most competitive Ivies - Harvard, Yale, Princeton) and have SAT scores of at LEAST 2100 (that is, no lower than 700 in ANY individual subject).
You should have at least one extra-curricular ("EC") activity in each of five categories: athletics (team sports, or if you're not athletic, some sort of athletic club will do: bicycling club, golfing club, etc.); academics (science bowl, physics club, mathletes, debate team, school newspaper or literary magazine, etc.); community service (key club, volunteer work, scouting, etc.); fine or performing arts (school play, band, orchestra, etc.); and school leadership (student council, student judiciary, peer leadership, etc.). Keeping the same ECs for all years of high school is more impressive than dropping old ones and starting new ones every year, as it demonstrates genuine interest instead of resume-padding.
U.S. universities rarely give scholarships to international students, and you probably won't "stand out" as much against Ivy League applicants as you would at other, less competitive, universities, Therefore, if you NEED a scholarship and want to go to the U.S., you should focus on non-Ivy League schools or scholarships available from your own country to attend any university.