The primary step even before deciding on the Institute is to determine which field, or branch appeals to you to the extent that you want to spend the rest of your life in that field. Once you have done that, the next step is to research which Institute specializes in that field.
Lastly, I will suggest you go for IIT Delhi without much hesitation as it is a complete package, with the much coveted IIT tag, the best faculties, labs, and other amenities.