Hello, a good ambition so you have to first know the basics of mathematics like algebra and then equations, logics then if you are cbse student you are lucky you can get a good basics in mathematics. Coming to sciences just be good in them because these are the basics for your +1 and +2 no need to prepare specially in science part.

hii sanjana'

its a great to know you are so intrested to hit IIT, since the question in JEE would be highly applicatios so as presently you dont meet that range , but make sure you be very good in basics , then i m sugeesting few books that will help you to understand easier 

books are The Feynman Lectures Volume 1,2 and 3, AP Chemistry by Zumdhal,

IIT Foundation Physics Class 8 by Pearson

IIT Foundation Chemistry Class 8 by Pearson

IIT Foundation Maths Class 8 by Pearson

IIT Foundation Science Class 8 by S.Chand

IIT Foundation Maths Class 8 by S.Chand

so feel which books are best and you need buy and study them up, 

coming to managing of schooling , where kindly build up in time management , since you need work on jee preparation atleat 1 year

regards