Unfortunately, there is no fixed weightage of topics in IIT JEE. It has always been this way. This is what makes it one of the toughest engg. entrance exams in the world. There are of course important topics in each subject.
Physics - Electricity & Magnetism, Sound, Modern Physics
Chemistry - Hydrocarbons, Thermochemistry, p block, Acid Derivatives.
Mathematics - Calculus, Coordinate Geometry, Vector Algebra.
Also, you can try a few things. Start solving the previous years' question papers. It's hard to observe any weightage trends in them, but still you'll able to understand and grasp the level of difficulty as well as conceptual basis of IIT JEE questions.
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