First I will suggest you to dat make a time table for urself and divide your time for all three subjects. Also join any coaching classes because as you are preapearing for IIT JEE then you will face a tough comptition. with coahing you can get guidance in a better way.
Always revise whatever is done on your tutions etc as it will help u remembering things. Stuff like test series and all is always helful. For chemistry, go through ncert for basic concepts first well then go for other books. Do practise of chemical equations.
And if are hving problem of time thn u need not worry about that now as can start online coaching where you can have a flexible timing ..So i wud advise you to join Askiitians.com.
Please feel free to post as many doubts on our disucssion forum as you can. If you find any question difficult to understand - post it here and we will get you the answer and detailed solution very quickly. We are all IITians and here to help you in your IIT JEE preparation. All the best .
Amit - IT BHU