You will be eligible to appear in JEE Main when you will be in class 12th. There will be two attempts for JEE Each year, one in the month of January and another in the Month of April. Next year In the month of September you can fill the form for JEE Main January attempt 2020. 

You have around 15 months for Your first attempt to JEE Main.

11th grade student not eligible to appesr for Jee Main. Only 12th grade appearing or appeared from recognised University are eligible . 

Maximum 3 successive attempts allowed. Since its two times year you can hence give 6 attempts in maximum .