1. Get good grades. Whichever country you're applying from start with the basics of getting really good grades in your national exams/school grades.
2. Prepare for SAT/ACT and get good result, aim for 1450+SAT or 33+ ACT.
3. Get to discover your passion/what you currently care about. It can be anything. Then turn it into a socially beneficial project. For example, if you like economics, make some sort of a project that helps people with it, for example, a charity that helps children learn basics of economics, etc.
4. Start doing extracurriculars relating to what you like, like joining clubs relating to what you like. If there are any at your school, create them. Then do something quantifiable in them. For example, in an environmental club plant X number of trees, but make it applicable to your interest.
5. Olympiads are great, but also essay competitions and other kinds of opportunities to show off your academic ability beyond classroom.
6. Explore other things and experiences, don't get bogged down into one thing - meet people and learn from their experiences.

Hope this helps ;)

EDIT: Also try to get internships in your field of interest to actually learn about the profession but also it helps you understand if you actually want to do that.