According, to the 2017 analysis you can check the important topics for JEE Main below

Physics

Capacitance- 6.67%

Current Electricity- 10.00%

Calorimetry & Thermal Expansion- 6.67%

KTG & Thermodynamics- 6.67%

Rigid Body Dynamics- 6.67%

Work, Power & Energy- 6.67%

Modern Physics- 10.00%

Chemistry

Chemical Bonding- 6.67%

S-Block- 6.67%

Alkyl Hallide, Alcohol & Ether (Reaction Mechanism)- 13.33%

Mole Concept- 10.00%

Mathematics

Co-ordinate Geometry (3-D)- 6.67%

Application of Derivatives- 6.67%

Functions- 6.67%

Matrix & Determinants- 6.67%

Probability- 6.67%

Good Luck!!