Which are the exams and what is their cutoff to get admission in top B-schools? What preparation is needed for those exams?

Exams for B-School admissions are CAT/XAT/SNAP/TISS/CMAT. Cut-off depends on the college you are looking for admission, it varies from college to college. No doubt aim for higher percentile to be on safer side. You can check on individual colleges websites and discussion groups to get respective cut