Hello Niyati,

The most important chapters by which you can score good marks in jee mains are 

In mathematics-

Coordinate Geometry (Straight line, Circle & conic section)

Quadratic Equation

Complex Numbers

Vector & 3-D Geometry

Sequence & Series

Binomial Theorem

Differential and Integral Calculus.

In Physics-

Electristatics

Current Electricity

Magnetic effects of current & Magnetism

Electromagnetic Induction & Alternating Current

Optics

Thermodynamics

Oscillation & Waves

Modern Physics (Dual nature of matter & radiation, Atoms & Nuclei)

In Chemistry-

Chemical Thermodynamics

Chemical Bonding & Molecular Structure

Electrochemistry

Bio-molecule & Polymers

Environmental Chemistry

Chemistry in Everyday life

p & s block elements

d & f block elements

(organic chemistry of class 12th most important as well as scoring) etc.

thanks for query

All The Best.