Even i used to get anxious about this fact when i was in 12! Dont panic just make sure your concepts are clear while preparing for boards because in mains they will not ask you any question which is out of  syllabus rather they will ask you very conceptual questions. You may join crash course from any known institutes just after your board exams. 

Good luck!

Hi, First dont panic. You can do it. I suggest that you study topics for Class XII and then solve JEE questions on the same. Check the previous year papers for the class 12 board exams also. You will get an idea of the questions and weightage. Dont know an answer from a question paper - check it up and learn. Dont mug up as if you forget one bit you will be lost. Instead you learn how to solve and the concept behind, you can always answer the question. One article that deals with preparing for class 12 and JEE is https://engineering.careers360.com/articles/how-prepare-for-jee-main-and-class-12th-board-exams. See JEE mains have to be cleared first so do that first. You will have one month's time for JEE Advanced once JEE Main is over. all the best!