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%

You should start your preparation with NCERT books as it will help you to make your concepts clear and it will build your fundamentals strong. It is very important that you have strong base to score good marks in your exam.

After NCERT, you can recommended below books

Physics
H.C Verma
Irodov

Chemistry
OP Tandon

Mathematics
RD Sharma
SL Loney

You can check the sample papers from the given below link

https://engineering.careers360.com/articles/jee-main-sample-papers

All the best for your preparation and exam!!