Hello,

You have to do UG and PG Computer Science Engineering. If you have done it then you can apply for PhD. There is also provision of Direct PhD which can be done after UG i.e. bachelor's course. It is good  for your career if you do it in top IIT campus. it will increase your recognition.

The learning path i.e. Interediate-> UG(CSE)-> PG(CSE) -> PhD(CSE). It is advised to do full time PhD course. it creates a good impression that you have worked hard and gave our best efforts and attention for 2 years to your interest.

Admission is given on the basis of merit. if a candidate holds a CGPA above 6.75 is eligible for the program for general category. for reservation category, 6.25 - 6.5 CGPA. There is no entrance exam for PhD admission.

For more details, please visit the link below:-

http://academics.iitd.ac.in/content/minimum-qualifications-admission-full-time-phd-programmes

Hope it helps!