If you belong to OBC or general category then you must score minimum 75% in aggregate in your class 12th ,if you are belonging to SC ST category then you must secure minimum 65% in aggregate in class 12th to be eligible to appear in JEE advanced, however students who are among the top 20 percentile of their board they are also eligible to get into IIT so if you satisfy any of about two criteria then you are eligible to take admission into IIT.

This is just 12th  criteria, you must to secure a good rank in JEE advanced to get admission into IIT.