From where and which chapter and how should I start my mains preparation for the best result??

In order to score good marks in your exam you should start your preparation through NCERT books so that you can build your concepts strong.

You can check below important topics which carries more weightage in the exam

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 (ReactionMechanism)- 13.33%

Mole Concept- 10.00%

Thermodynamics and Thermochemistry- 6.67%

Mathematics

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

Application of Derivatives- 6.67%

Functions- 6.67%

Matrices & Determinants- 6.67%

Probability- 6.67%

Good Luck!!