Hii,

Generally registration for class 12th is done when you are in class 11th.

You must be eligible for board exam if:

• He or she should studying in a school recognized or affiliated to the board or any other recognized board of secondary education in India.
• He or she should pass qualifying or equivalent examination so as to get admission to that class. 
  
• He or she should produce the school leaving certificate or transfer certificate signed by the head of the institution.
  
• Document or certificate of the qualifying examination.
  
• Date of the birth certificate.