I am aiming to be an JEE/IITian, but I seem to be weak in chemistry. What should I do?

Always remember, all three parts of chemistry are interconnected, you can'i ignore any part. You should have strong hold on each part if you wanna score good marks in this subject. Start with Physical chemistry and only after that start inorganic and organic section. . Try to understand complete mechanism behind each and every reaction. Specially for organic chemistry this is very important to completely understand mechanism of reactions. In physical chemistry, be thorough with all the formulae. Solving a lot of problems in physical chemistry helps. For inorganic section give extra attention to d,f-block elements and co-ordination compounds. Most of the questions that come from p and s block elements are easy to solve. I will suggest you to pay extra attention to co-ordination compounds and d-block elements. ALL THE BEST!!

I think if you go thoroughly and clear your conception with regular practice there is no reason that you can't carry decent marks in Chemistry. You must be aware that Chemistry is a scoring subject.