Dear Shimran, 

If you have given your 12th Examination this year and have attempted first time JEE Mains and JEE Advanced, then you are surely eligible for next year to appear forJEE Mains and JEE Advanced.

And you must also satisy this condition to get admission in NITs and IITs. 

JEE Main is applicable for admission to NITs, IIITs and CFTIs participating though Central Seat Allocation Board 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.