what are the easy enough chapters to read, that are sufficient to score at least 120 in JEE MAIN?

To score good marks in JEE you should follow below Prioritizing chapters-

Physics
Physics requires you to have a strong hold on concepts. Once the concepts have been mastered, they are very easy to retain. Some chapters for JEE from physics have a heavy weightage in the exams. Heres the priority sequence you can follow chapter-wise.

Mechanics
Any JEE level mechanics problem can be solved by a combination of one or more of these:
Newtons laws
Work energy theorem
Law of conservation of energy
Law of conservation of momentum (linear/angular)
Equation of continuity
Bernoullis theorem

Electricity and magnetism
Thermal physics
Fluids and waves and sound
Potentiometer-related experiments
Optics
Modern physics

Chemistry

Physical chemistry
Mole concept
Atomic structure
Other topics-  Other concepts like Chemical Thermodynamics, Chemical Equilibrium, Chemical Kinetics, Electrochemistry, etc. have nearly equal weightage. So, they can be revised in any order, but remember that a few questions are guaranteed to come from this section.

Organic chemistry
General organic chemistry
Named reactions
Organic synthesis

Inorganic Chemistry
All the chapters are important

Mathematics

Calculus
C0-ordinate geometry
Vectors
Algebra
Trigonometry