First off, it's great that you're already thinking about this, as you need to start early!
Here is what I learnt during the short amount of time that I looked at American universities;
Look for one with needs-blind admission. Off the top of my head, Princeton is one. Essentially means that if you're good enough they'll accept you, if you can't pay then they'll make up what you can't. They will also help you find a job on campus to support yourself. Princeton does actually offer this to international students. Not going to lie, getting a scholarship would be really hard, so look at needs-blind admission too.
If you're heading to the US, go for the very top. Imo, it's not worth spending that amount of money to not go to an Ivy League or equivalent college. Sure, the experience would be great, but for that amount of money, going to a college that's only on a par, not even as good as, a top UK uni? Don't do it!
Admissions- you'll need SAT/ACT and A-Levels (and do well in them too). I went to a Fulbright Commission talk and normally the acceptance rate for Ivy League is about 6%. Because you're an international student, you can halve that to 3%.
Also, US colleges are big on all-rounders who do tonnes of extra-curriculars. Are you on a sports team? Think about joining your school magazine (if you have one)- contribute to school life!
Why do you want to apply to American universities? Liberal Arts course? The experience? You think you'll get a better job at the end? There is no right answer, but do think long and hard about this and whether or not you're applying for a suitable reasons.
Make yourself known to the colleges you want to apply for now. They will take a note of who attends various talks and open days and contacts them over email, and if admissions tutors recognise your name when they see you're application you've already got a head start.
Urmmm, I think there was more stuff but I've just had a mind-blank. I may be back to edit this, and if there's anything else you want to know I'll try my best to answer it
In the mean time, you can check out the Fulbright Commission website.
Also, if you think about it and realise maybe you're not up for American colleges, then think about applying to UK universities that offer a year abroad in America. That's what I'm doing now