How to apply for engineering through online application in tamilnadu?

You can fill the form given by Anna University – Chennai, then based on your city of marks, you will be called for counselling and based on your interest and marks you will find a good college. All the best.

I don't know about other states ,but in Gujarat its by ACPC ,they publish a book & pin.then you have to register on there website & book carry all the information that you will need to move further