What should be my study plan 12th and JEE preparation?, I join FITJEE in class 12 .

You can follow below strategies-

1-While JEE Main is about practice, board exams are about theory. You have to be at articulating the theory and concepts you have learnt rather than applying them across many questions. Until you have a good grasp on theory & concepts you can do well while practicing for JEE Main. You would be tested heavily on theory and hence this provides a great opportunity for you to get well versed with it and brush up some concepts in more detail as it would ultimately help you in JEE Main as well.

2- In board exams, revision is the key to scoring marks. Its also important for JEE Main that you remember all the formulae so that you don't miss on the easy questions.

3-Having a thorough understanding of the question paper pattern is a must. JEE Main follows specific paper patterns that can be understood & strategically used to score well.

4-Practice previous year test papers of JEE Main will help you polish concepts that you are not confident about and get into the real groove of the JEE Main.

5-Studying with the help of a test series will also help you focus, understand your weak points, and augment your studying methodology accordingly.