If I complete computer science engineering then what I select to study as post graduate
There are many options you can opt after B.E/ B.tech Mechanical Engineering, It depends on your interest & Potential. If you are interested to go for further study, After completing your B.Tech studies. M.Tech is the next step that allows you to select your field of expertise and excel in it, Specialization in a particular field provide you an edge during job interviews.
The other option, If you are interested for MBA programme then Some of the specialization like HR, Marketing, Digital Marketing, International Relations, Sales and more are some of the well-known and preferred specializations.
You can go as per your interest.
Good Luck!