I'm doing 12th now.i didn't start my iit preparation yet. how should i do my preparation?,which chapter i should focus now in phy,maths,chemistry?

Dear Vijai ar 

You are  still not late but you have to put some extra efforts towards your studies. you should focus on following topics 

 In physics -mechanics, Optics, ELectodynamics, Modern physics,Thermodynamics, Waves and sound,Fluids.

In chem- Organic Chemistry, Ionic Equilibrium. Chemical Kinematics,Coordination chemistry, metallurgy( IIT interested from few years),In organic Chemistry.

In maths- Vectors , 3d Geometry, Calcalus, Probablity,Trigonmetry, Complex nos,( Dont waste  more time in BInomial theorom and P&C).

Hope this helped you

Best of LUCk.