This should be moved to the "Studying in America" sub-forum, which itself is in the "International Study" forum. Alternatively, you can make a thread there.
You want to do the SAT Reasoning Test and two Subject Tests. The GRE - frankly, I don't know how the name did not give this away - is for graduate admissions, ergo, for graduate school. That requires already having a bachelor's degree.
Most colleges who do give substantial financial aid are need-aware, meaning that before giving you an admission decision, the ad-com will check how much you can pay. If they want you bad, it's cool, otherwise, if you can't pay too much and they can take another applicant instead of you, that's enough grounds for rejection at a need-aware college. Some colleges are need-blind but are not full-need, like Cornell, meaning that you can get accepted and be offered no financial aid at all! Six college are both need-blind and full-need. These are MIT, Harvard, Yale, Princeton, Dartmouth and Amherst. All are highly selective. Actually, competition will be fierce for you if you're applying for aid. You'll competing with other applicants just like you, who want to study in the USA.
Do your research, figure out what colleges provide aid for international students, read into them and begin your application. You should use the CommonApp, and not UCAS! Some colleges have their own application system, like MIT. Remember that colleges are looking to build a community and if you can be an essential part of that community, you've got a good shot. The catch is that varies from school to school and also depends on the other applicants. Show them the best version of you. What is the "best" version of you? That's for you to decide.
Again, go do your own research. There's only so much people can tell you.