If you have completed your 12th board exams in 2017, 2018 or 2019 then only you are eligible to take admission in any NIT/IIIT. If you were pursuing some other diploma/degree course after class 12th but you want to leave it then that is also fine as long as you satisfy the above criteria.

The college might ask you to bring a migration certificate from the institute last studied at so keep that in mind.