For studying BBM, you should have studied your 10+2 with commerce subjects as your core subjects. So, yes you can study BBM after your Intermediate. Different Institutions have different criteria for admission. On an average, you should secure a minimum aggregate of 50-60% in your 12th to be eligible for BBM. Few Institutions conduct Entrance examinations for admission to their Institutions and few Institutions conduct admissions based on merit secured in their 12th.