Hi Vaishnavi, its good that you are going to prepare for GATE from 2nd year.

First of all you should check the syllabus for exam a some topics are removed from syllabus in coming years.

Now the books you should refer are:

Operating system: Galwin is best book. Others are Tanenbaum, William Stallings and lectures of NPTEL.

Theory of Computation: Introduction to Automata Thoery by Ullman

Algorithms: Cormen

Computer Networks: Tanenbaum

Computer Organisation: Carl Hamacher

Database Mangament System: Korth

Compiler Design: Ullman

Digital Logic: Morris Mano

Practise more number of previous year questions which are the key to success. Attempt test series.

If you attempt, you will surely score higher.

You can also join any coaching institute and get their material and teachers but you have practise more by yourself. 

• Self Study
• Previous year question paper
• test series

In the exam year, revise your syllabus in the month of Jan, don't skip that.

Best of Luck!