To be eligible for competitive exam you can write on the basis of your class 12th or after completing your graduation or in final year.

You can start your preparaton from now and you can give your entrance exam in your final year. You can start your preparation for GATE or CAT exam if you are interested.

If you want to write exam on the basis of class 12th you can go for UPSC exam.

You should always prefer a course according to your interest. So, choose your decesion wisely.

Good luck!!