Important topic 

• Discovery of electron, proton and neutron:Sometimes theory  questions are asked in jee mains.
• atomic models:dalton's atomic model and its postulates,Ruther ford's atomic model and alpha particles scattering xperiments, Bohr atomic model and its postulates are very important
• dual nature of matter:practice numerical problems.
• expresssion and formula for the enrgy of electron, radius of electronic shell,frequency and wave lenght of light during transition of electron and angular momentum of electron in an orbit are fequently used to solve numerical problems.
• quantum numbers
• shapes of s, p, and d orbitals

Hello Dear,

first of all atomic structure chapter is the less input chapter and high weightage.

https://www.embibe.com/exams/jee-main-important-chapters/

for atomic structure:
https://www.askiitians.com/iit-jee-atomic-structure/preparation-tips.html

good luck