Since I went through this phase,I guess I am a perfect fit to answer this question.Well, first of all let me tell you that , everyone has their own formula or strategy to crack any exam.I will take two assumptions.

If your are in class 11:

First form the foundation by gathering basic concepts.Practice RD Sharma for mathematics and try solving NCERT thoroughly for all subjects.Preparing for Jee at this stage is not manadmand,you can if you get strong holds on basics.Besides you can just go through the concepts of Chemistry specially now for Jee since it is the most scoring part.

If you are in class 12:

Starting from the academic year,you need to do parallely both Jee and Boards.Solve Jee questions from class 11 parts and at the same time study well for Boards (class 12 part).Divide your time in this Strategy.At the middle of class 12,solve both 11 parts and 12 parts for Jee part.I will suggest do self study since it is the only weapon which will be with you.Still if you live close to any coaching centre you can get admitted.Solve ample number of Mock papers for Jee and last 10 years for class 12 Boards.

Hope this helps you mate.All the best and during this preparation stage,stay fit and don't engage much in social media.

Basically, you should focus more on boards rather than mains now. Even the smaller chapters of physics and chemistry like biomolecules and polymer can fetch you more marks in Jee mains if you revise it well for boards. Moreover, preparing for boards currently will improve your basics and will brush up your concepts.