I have completd MBBS , now i want to do phd in USA, how do i go about?

Get a list of universities that offer doctoral programs in your field. Visit their websites and learn about doctoral admission requirements. PhD admissions are usually very selective and requires that you have a few research projects in mind. The best thing you can do is to contact the admissions director for doctoral programs in your field along with some professors who share your research interests since most of the time your admission depends on these professors interest to work with you.

Good luck!