I am from Iran and I am 18 in third grade of high school and planning to emigrate to America. The actual problem is that I can't afford to apply for university there, I was hoping that I can go there for study and work to save the minimum money, so what is necessary for me to go there? Do I need a very high score in high school? How can I get scholarship? Do I need to be a part of a specific science activity for getting the scholarship?
You get scholarship by qualifying for it. Different scholarships have different qualifications. Some (many, if not most) require academic excellency, i.e.: you must be a very good student. But some are based not only (or at all) on your achievements but (also...) your background, your community affiliations, etc.
In addition to the excellency scholarships, you may want to check out the Iranian diaspora organizations that may want to support you, or specific scholarships for specific areas of study (or geographical areas).
To get to the US as a student, you must be able to show that you can pay for your studies and living expenses without working. It is illegal to work as a student, unless certain conditions are met (up to 20 hours, on campus, with the school approval, as long as you're still a full-time student in good standing, etc - not something that you can do to pay off your studies and living, more like ice-cream and movies spending).