There is very limited financial aid for international students - realistically you would need to target the need-blind universities only, which is Harvard and it's ilk primarily. You would need to be at or above Oxbridge level for this, and you would need Oxford level GCSEs as well, due to how they assess applications. I would recommend looking into this as a starting point, before getting ahead of yourself otherwise as this is a very large barrier to entry, realistically.
Beyond that you'll need to take the SAT or ACT, and this is more important for some colleges than others - gradewise you'll need to be averaging mostly As between GCSE and A-level at least. Moreover, extracurricular activities are a very big deal in the US - and not just the "supercurriculars" Oxbridge likes here. They very much want people who are the captain of the sports team, debate team, volunteer, do all this and that. There is some reasoning to this, as when applying to the US you don't apply to a specific subject (normally - there are a couple of exceptions, mainly for engineering) - you just apply to the college "at large" and you'll declare your major later. Thus they want to see you can excel in pretty much anything you do, as you might do...anything, when you're there.
For smaller/lesser known colleges this is not such a big deal, but you are MUCH less likely to be funded at one of these so...you end up back at the starting point.