First set a target, whether you need to take GATE for job opportunities in PSU or enter an IIT for your M.Tech. In both cases you need to have a top rank based on your category. Then download the syllabus, read it carefully, find the topics that interest you or which you already know well. Prepare a plan according to the interests in the subjects. Start studying the basics with standard books. After completing a subject, try to solve the questions for that subject from previous years question papers. You will be able to solve some questions, develop an eagerness to correct your concepts in the questions that you were not able to answer. By the time you finish a subject, you will have a mind set to start another subject. And then repeat this for all the subjects. Finally take online test series from leading coaching institutes and test your level with other students all over India, correct your mistakes. You should be able to score very good marks.