to score good marks what i can suggest you is that at first you need to be clear with the basic concepts and be thorough with the NCERT books as most of the questions will be asked from NCERT only.  For Maths, the more you practise the better you get at. If you have practised enough the sums can be done quickly and correctly. I can refer some good books for maths such as differential calculus by A Das Gupta, Trigonometry, Geometry books by S.L. Loney, Higher algebra by Hall and Knight, etc. In case of chemistry, most questions are asked from organic part. Some good reference books for chemistry are organic and inorganic chemistry by Arihant Prakashan, Concepts of organic chemistry by MS Chauhan, organic chemistry by Morrison and Boyd. For physics, many students find it difficult. But it will be good for you if you start with some chapters such as work power, friction ,centre of mass , surface tension and rigid mechanics. Stick to it, give some time. Clear all your doubts. Once you are clear, you will not find it difficult. Try and practise the mathematical parts of physics and refer some good books like HC verma and IIT JEE Physics by DC Pandey, IIT JEE by Resnick, Halliday, Walker etc.

For jee main topics click on the link below - https://engineering.careers360.com/articles/jee-main-syllabus-weightage

Hope it helps you and all the best with your preparation

Every chapter is equally important. 

The questions would come from which chapter in JEE Mains cannot be predicted as it could be a question asked from combination of 2-3 chapters. 

For example it could be a combination of functions, limit and differentiation. 

If you want whole syllabus or have any other queries do mention it in the comment section down below.