To qualify JEE Main you must score cutoff marks decided By NTA. every year it is close to 100 out of 360. Once you qualify JEE Main you will be able to take admission into NITs and further you will be eligible to appear in JEE Advanced. If you qualify JEE Advance then you can take admission into IIT.

The eligibility for JEE Main:

You must be a student of Science stream with Maths and Physics as the subject and You must be Eitherin class 12 or passed it in 2017 or 2018.

All 12th grade appearing or appeared in last year and who have passed all subjects from recognised University are eligible . Studnets must have PCM as mandatory subjects . Students from open boards like NIOS are also eligible . 

Entire eligibility criteria shared below. 

https://engineering.careers360.com/articles/jee-main-eligibility-criteria/amp