Hello aspirant, hope you are doing good.

So, as you are currently in class 12, you cannot appear for JEE Mains right now, You need to first pass your class 12 exams, then only you can sit for JEE Main exams. And even if you don't sit for JEE Mains in the year you passout, your first attempt will be over. Which means, if you don't appear for the JEE Mains in 2021, you will have 2 more attempts left.

Hope this helps you. Good Luck!!