Just start with knowing the syllabus of your subjects, that u have to prepare. Start from the beginnning and dont do selective study as it never helps. cover all the topics without leaving a single topic. Ensure that u solve as many problem questions as u can, this will help u gain tricks to solve similar questions in the entrance. In case u are looking for books, i will suggest u to go by Made easy publication and GK publications. This will solve your need of previous year as well as mock tests.

GATE exams need 5 to 6 months of serious preparation after acquiring the basic knowledge of Engineering.I belive its little early for you to think about GATe rather you should concentrate on Semester subjects