Hi, here eligibility criteria is mentioned as given on official site.

Eligibility for Admission to NITs, IIITs and CFTIs participating through Central Seat Allocation Board Admission to NITs, IIITs and CFTIs participating though Central Seat Allocation Board will be based on All India Rank as explained above in section 2.10 subject to the condition that the candidate should have secured at least 75% marks in the 12th class examination, or be in the top 20 percentile in the 12th class examination conducted by the respective Boards. For SC/ST candidates the qualifying marks would be 65% in the 12th class examination.

You must have completed +2 then only you can appear for jee mains.