In order to take admission in IIT Madras you have to appear for JEE Advanced exam.

To appear for JEE Advanced exam you have to qualify for JEE Main.

You need to score 81 marks in JEE Main general category to get qualified for JEE Advanced based on 2017 cut off.

To be eligible for IIT Madras you should have 75% marks in your class 12th or you should be in top 20 percentile. For SC/ST you should have 65% marks.

You can check the cut off from the given below link

IIT Madras cut off

Good Luck!!