In the order to become a CBI officer after B.Com (Computers) you need to appear in the combined graduate level exam conducted by Union Public Service Commission Examination. as per your request here I am giving you the details regarding to the entrance examination:

Eligibility:
The applicant should have completed Graduation with a minimum of 55% marks from any recognized university of India.

Nationality:
A citizen of India, or
A subject of Nepal, or
A subject of Bhutan, or
A Tibetan refugee who came over to India before January 1, 1962, with the intention of permanently settling in India, or
a person of Indian origin who has migrated from Pakistan, Burma, Sri Lanka, East African countries of Kenya, Uganda, the United Republic of Tanzania, Zambia, Malawi, Zaire, Ethiopia and Vietnam with the intention of permanently settling in India.