...medicine in USA?If yes what to do after sat? when to give MCAT? Right now i am in twelth standard and wish to study medicine in USA.If i need to give the SAT then after SAT do I have to take admission in a university?

Yes you will need to take the SAT for the application process to the university.
You might need some other tests as an international student, Im not positive.
Colleges normally have entrance exams, they are basic math/english classes to see where to place you. If you don't do well, you will need to take a remedial class.
The MCAT is normally taken in the junior year of college.

Yes you need to take the SAT. I got 2 perfect scores in math and reading, but my writing score was only 720, for a total of 2320. I only read this SAT guide, and that is how I got such high scores:
[Only registered and activated users can see links. ]
I recommend this guide to anyone seeking to get a higher score to get into a first choice college.