Hey Sushil ,
Greetings!

To pursue Phd. in Computer Science you will have to apply to different colleges and you will have to sit for entrance examinations according to the colleges .
Later you have to appear in the GD/PI round for the final selection.
If you have a particular college in mind kindly revert back so that I can guide you better.

Hope this will help . Good Luck ! You can reply to this comment if you have any further queries , I will be happy to help .