To decide which concepts are more important than the others that experience comes from solving previous years question papers. While solving previous years papers, you figure out which chapters get more weightage in the examination, how many questions come from each chapter and then according to that you can study.

I did this analysis myself of which chapters are more important from previous years papers.

Ankit Kumar Misra
JEE 2019 AIR 13, IIT Bombay CSE