Hello ,

As you have taken CSE as your engineering subject it is all about coding skills and programing skills so you will need to develop those in you if you don't have.You need to have a good command in programing subjects like python java c and c++.from the first year itself.The core subject in btech starts from 2nd year but then also you can start working from 1st year itself.

For gate preparation you can start from 2nd or 3 rd year itself.You need to take coaching either in an Institute or you can buy online lectures.Its all upto you.you need to be hard working and stay focus about your goal than you can achieve it 

Thankyou