important topics of jee mein 2020 january examinations

The important topics for physics are- EmI, Ac, Magnetism, Electrostatics, current electricity, oscillation waves and obviously optics.
The important topics for chem is inorganic and organic chemistry, inorganic being the kost scoring part. P block is the most important chapter. The whole organic part is important.

For mathematics,
Calculus, Vector and 3D, probability are the most important ones. Remember, in mathematics there are scoring chapters like Differential equation and matrices.. do not leave them. You may refer to this for details-
https://www.google.com/amp/s/engineering.careers360.com/articles/jee-main-2020
Practice the previous year papers well.