Hello Sanjeev, 

For appearing in jee mains exam student needs to qualify 10+2 with subjects physics, maths, chemistry.

And maximum number of attempts required are three weather they are taken consecutively or in years. 

As per the notification, for candidates to qualify for admissions to NITs/IIITs/GFTIs, they should have secured at least 75% marks in class 12th exam or be in the top 20 percentile in the class 12th exam conducted by the respective boards. For SC/ST candidates, the eligibility criteria is 65% marks in class 12th.

And for more information please memtion the percentile your asking about. 

All the best.