Start the preparation from TODAY onwards. Firstly, I appreciate your daring because you are going to take one of the toughest exam IIT-JEE. The probability of getting IIT is 8 among 1000 students. Everyone dreams to go in IIT but only few students can do it. Here The main point is how will i start preparation?? DEDICATION+VERTEX and nothing. Think about the future after getting IIT.
(1). Firstly, Avoid the social media like Facebook, whatsapp from now. Use Internet only for education purpose. (2). Go through the entire syllabus from CBSE website and check the weightage of topic. (3). Check the previous year question paper by topic (no need to solve)with VERTEX because you will get the idea about toughness level of question paper. (4). Each and every concept should be cleared.
(5). Now Solve the minimum 30 question for each subject(PCM) without looking the solution first that you read today by teacher or self. (6). If you will able to solve question then you will gain confidence and I'm sure you will adapted in the study situation so do a lots of practice. Lastly, believe in yourself. Focus on your strength rather than weakness. And one more thing, study with time management & proper way is more important than others.