During B.Tech u will have to give GATE (Graduate Aptutide Test) for admission in IITs for M.Tech ,then you should give CAT (Common Aptitude Test) to get admission in IIMs for MBA, and GRE (Graduate Records Examinations) for graduate courses in USA and there are many exams such UPSC,ISE etc based on your interest of field.

This exams can be given even after graduation but it is beneficial to give this during graduation.