You should write the first exam in your fourth year B.Tech !
Hey there! I am a GATE aspirant and currently in second year B.E as well. I approached lots of seniors who had cleared the GATE exam from my college and online aswell. And here is what I was told, First off you should know the complete syllabus of GATE exam, by complete I mean complete! with weightage and important topics. And never keep General Aptitude section, and Engineering mathematics for later study because both of them needs some momentum to score. let me explain. General Aptitude includes Grammar, Vocabulary, Sentence corrections, Critical reasoning , numericals on speed-dist-time, cost-profit, direction problems etc which requires your brain to be trained for sometimes, so if you start learning 5 new words everyday, and 30 mins for numericals then imagine how good your vocabulary would be! and you will get enough time to concentrate on your technical subjects. Same way you need to remember lots of formulas in mathematics and if you start solving some problems and noting down formulas and remembering them, then out of 100 marks in GATE, 30 marks can be scored easily. Again, scoring in General Aptitude and Engg. mathematics is easy and you should start preparing from now itself.
Coming to technical part, make sure you know every concept and try to get your doubts solved then and there itself and join some reputed GATE coaching institutes( sometimes they give a test and if you clear it then you'll get discount in your fees).
Suggested book for increasing vocabulary- Word Power Made Easy, GRE high frequency words.
Suggested book for numericals -R.S Agarwal.
Suggested book for Engineering Mathematics- B.S Grewal
Link to GATE 2017 Syllabus- gate.iitr.ernet.in/wp-content/uploads/2016/07/Syllabi_GATE2017.pdf