Ace your GCSEs, I'm sure you'll do great!
Register for the SAT for early year 12, I think there's one in October. This is probably the best time to do it as the workload can be overwhelming after the initial months, so studying for the SAT doesn't help (I did my SAT a week ago, right in exam season). Use
Khan academy to practice, they have lots of personalised help and practice exams.
Look at the avg. SAT scores for the schools you want to apply to, so you can aim for this. Of course, you can take it more than once if you feel like your first score isn't good enough.
Schools like MIT want 2 subject tests, so make sure you register for them when you feel ready (You can't do them on the same day)
Try to keep your grades high throughout year 12. American Uni's tend to look at transcripts, so your midyear reports etc.. and not just your end of year grades. So make sure you keep up with the course load and revise effectively from the get go.
Other than that, as mentioned above, extra-curricular is very important. Use your long summer to do some volunteering etc.. programs like NCS are probably really good too.
Try to do stuff outside school that shows your interest in whatever you want to major in. Many schools don't just want a smart student, they want someone who is interesting.
I hope this helps! If you need any more help just message me! I'll be applying for about 6 American Uni's this year.