Hello there

Basically all are important but if you are running of time then you should focus on the most important ones to score good. So for that you can study the chapters which I'm listing below.

Chemistry- Periodic Table , Coordination Chemistry , Chemical Bonding ,Stereoisomerism , Electrochemistry and Ionic Equilibrium and thermodynamics also.

Physics- Kinematics, Heat and Themodynamics, Optics, Electrostatics, Modern Physics.

Mathematics- Coordinate Geometry , Calculus(Both differential and integral), Complex numbers & vectors.

You can refer this link for more information:

https://engineering.careers360.com/articles/jee-main-syllabus-weightage

I hope it helps you!

Good Luck!