Help me in GATE preperations?

I'm currently doin my 1st year B.E. Mechanical Engg. under Anna Tech,Chennai. When shud i start preparing 4 d GATE xams? I'm interested in joining M.Tech. Aerospace Engineering in IIT Bombay, IIT Kanpur or IIT Madras. So wat rank shud i get 4 dat? Is it necessary 2 take spl coaching? if so, suggest me some excellent coaching centres in chennai. n suggest me some good books 2!!!