Hii aspirants 

Let me clear your doubt. 

In gate the question comes from the what you study in 2nd 3rd and 3th year. Since now you are in 2nd year  3rd sem you dont know even basic of topic. So first of all study your subject topic throughly so that it will be helpful for you to solve gate question easily. So to start preparation of gate 3rd year is best. But one more thing you can do that atfirst see the slybuss of gate and try to solve the problem of  the topic which you have studied in 3rd sem. 

But i will suggest if you want to join coaching for gate preparation then join in 3rd year. 

I hope my answer will be helpful for you Good luck