Dear Biswajeet,

Both the classes are important to prepare for JEE Main and holds somewhat same weightage. 

You can check the weightage of marks of 11th and class 12th below

Class 11th

Chemistry- 56
Physics- 52
Mathematics- 60

Class 12th

Chemistry- 64

Physics- 68
Mathematics- 60

To score good marks in your exam you should focus on the topics which carries more weightage in the exam.

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!!

hey,

According to previous years, 12th have 60% wiehtage an 11th have 40%. But don't go with only 12th or 1th, not all the question are answered. My advice is to read the theory of chemitry of both 11th and 12th, formulas of maths and do concepts of physics.

good luck