That may depend on the university you're applying.
But most if not all universities in the USA ask for
SAT. So i'll advice you to sit for
SAT1 and
SAT2. It's syllabus is not like A-levels but you'll be able to cope with it easily especially for your subjects.
You may sit for it during your holidays after your exams or even before. If you want to register for SAT, click
here.
Here are some info you might necessary :
http://www.internationalstudent.com/study_usa/ -----> You also have a guide on the top right
http://www.usastudyguide.com/As for the subjects, I think that Maths, Physics and Chemistry should be enough. Biology would also be a plus for you.