You need to appear IIT JEE which is a national level exam If you want to get admission in top colleges like IITs, NITs, IIITs, GFTIs & you should have minimum 75% in class 12th as a general category & 65% as a reserved category to apply for the exam. And if you are interested in Tamilnadu colleges then mostly the colleges takes the admission through TNEA, the admission criteria varies college to college so if you specify your desired colleges name except above given names then I can help you more.

Good Luck!