First of all, you must have to clear all the basic concept. Then you have to practice a lot questions of all the subjects without looking at answer. while solving problems, you have to generate so many questions from question also so that it might be helpful to clear your fundamentals. There are so many gate coaching institutes which organize online test series that you have to buy it and practice with it. There are so many online portals available where you can discuss all your problems or doubts. Good Luck!!!