Amit, admission to Top Institutions like IITs, NITs and GFTIs happens through JEE Advanced. Now JEE Advanced is the next step after JEE Mains and for eligibility to appear for JEE Advanced, you should have secured a minimum aggregate of 75% in your 10+2.  So, for being eligible to seek admission in IIT Madras, you should target 75% in your 12th. For more information in JEE Advanced, please visit our page at:

https://engineering.careers360.com/articles/jee-advanced-eligibility-criteria