You should always try to complete your whole syallbus for JEE Main and focus on important topics to score good marks in JEE Main. 

If you have less time then you can leave the topics which carries less marks in JEE Main exam

According, to previous year analysis you can check topics which carries more weightage in the exam. Focus on these topics.

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!!