According to me your main focus at this moment should be your board exams and if you are in CBSE board then it will be a bonus as a huge part is common for both the exams. In board exams, revision is the key to scoring marks. It's also important for JEE Main that you remember all the formulae so that you don’t miss on the easy questions.

NCERT books are known for being the best while preparing for JEE Main and 12th board exams.

Along with studying the syllabus, practical studying is also a good option while preparing for JEE Main and 12th board exams. Sample papers and questions banks will help you know how the exam pattern will be. 

For more details go through the following link:

https://engineering.careers360.com/articles/how-prepare-for-jee-main-and-class-12th-board-exams

Good luck! 

• You should go through with the concepts first because if you are conceptually strong enough then your hardwork required for getting the marks will be somewhat less.
• Do the NCERT first and then after go with the other reference books .
• Do as much problem of every topic as you can and try to do the problems as much as you can.
• After the completion of your NCERT , do the previous 10 years papers of JEE mains which will help you a lot to understand the whole scenario of JEE mains.
• Lastly, believe in yourself to succeed.

Firstly Do Motivate your self to crack the exam, As you told that you are in class 12th and Jee Main syllabus is class 11th & 12th based, If you are well prepared with that, It will be easy the further study. The exam is easy to crack if it is planned & followed properly. Here are some preparation tips you can follow:

Work on a Time Table: After carefully checking the syllabus of JEE Main work towards creating a perfect timetable which will fit your schedule while leaving enough time to prepare for the examination within the days left. Make sure to add short and sweet breaks in between for some relaxation.

Make your Basic Concepts Strong: While it is essential to study the topics mentioned in the official syllabus, it is also advised to study the basic concepts well first. By knowing the basic concepts of the units involved in the examination, you will be able to understand each topics well and in depth. This will result in a greater understanding of the topics and will make it simpler for you to study complicated concepts present in the syllabus.

Improve your Weak Points: Stressed over some topics or formulas which you just cannot seem to understand? Give more time and focus on these topics to improve it. Keep small notes and constantly revise those concepts which has been hard to understand. By constantly studying the topics you find difficult.

Write Notes and Prepare Flash Cards: Like discussed in the previous strategy, keeping notes greatly helps during the preparation process. Notes written in small diaries are highly accessible and you will be able to study from them anywhere anytime. By writing down those complicated theories and formula and revising them, you will be able to remember and understand them more efficiently. Similarly, create flash cards to help you with quick studies and revision.

Attempt Mock Test and Solve Sample Papers: Mock tests will give you an almost real experience of how the examination will be. By solving mock tests, you will be able to work on time management while at the same time understanding the question paper pattern.

Revision: While studying it is important to revise those topics which has already been studied. It is advised that each day, a certain amount of time should be divided for revision purposes. Revision is very important as the syllabus is quite vast and all topics have similar importance level which has to be remembered and known by you.

Good Luck!