some important chapters of math we should focus on to get decent marks in jee main mathmatics is

Dear student focus more on calculus which would help u to crack the mains atleast 5-6 questions would be ask there but if u wanna crack the mains study the well through time

If you are really wanted to score good in mathematics than cover all the topics and concepts.
Because you can not depend upon only some of the main topics.
It is not necessary that you will be able to solve all questions of that topic in exam because surrounding and the situation are very different at that time.
So it is better to prepare yourself for any bad situation.
And only the way is you are prepared with all the concepts.
And it is not necessary that they will give question from that concepts they may vary the question and percentage of topics.
But you must cover these topics.
Like
Algebra
Geometry
Calculus
And statics
Good luck.