Hello Ramit,

JEE Main in a all India level engineering entrance exam conducted by NTA (National Testing Agency). The exam will be conducted in two sessions - one in January and the other one in April. You can sit in JEE Main both times i.e. in january and also in april but for that you have to first be in your 12th class. So you will be giving the first exam in january that is before your 12th boards and the second exam in april after your 12th boards. Through JEE Main you can get admission into NITs,IIITs and GFTIs. Also JEE Main is the qualifying exam to sit in JEE Advance. For JEE Main january session, the application form will be available in the first week of September of the previous year. The April session of JEE Main will be conducted is the second week of April. 

You can also go through this link and checkout the details about JEE Main : https://engineering.careers360.com/exams/jee-main

Hope this helps. Good Luck !!!