how should i start my GATE Preparation?

First, make a proper schedule before starting the preparation of the GATE.
Go through the syllabus. Group all the relevant books for the subject. They should include the books for Fundamental and basic concepts as well as for specific problems, guidebooks for GATE and previous year question papers.
Always follow standard books for GATE. Expertise in whatever portion of syllabus you practice.
Make notes for faster revision.
Go through the previous year question papers and try to solve them. This will help you know where you stand.
Start preparing from the initial topics i.e. the basic ones and note down the basic concepts (definitions, unit, dimension etc.) as well as necessary theories, formulae etc.
Try to apply your own logic and tricks in solving problems and note it down so that you don't forget them because it will save your time in the examination hall.
Finally, go for tests based on the whole syllabus and if necessary take help of others in conducting tests.