Feeling sorry to say, but you are not eligible for the Central Bureau of Investigation after completing 12th the basic requirements to join CBI is graduation. In the order to join the Central Bureau Of Investigation department you need to appear in the Combined Graduate Level examination conducted by Staff Selection Commission.

Eligibility
The applicant should have completed Graduation with good academic record from any recognized university of India.

Age:
Age is between 20-25 years.

Physical requirements:
Height:
For men: 165 cms.
For women: 150cms
Chest: 76cms with expansion