What must be my score in CBSE class 12 board exams and what are the other competitive exams should I take to get into top colleges around TN
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!