How to prepare for GATE exam?

Hi,I just finished my 12th,going for engineering,I got average marks,so can't get into best colleges.I wish to go for M tech program after B tech.And I want to do M tech in IIT.So please give all the information I need.Do I need study 12th portions so that I can perform better in GATE?How to prepare?After all I have 4 years to prepare.Is there any other exams like GATE,for M tech?