Hello there,
The basic eligibility criteria for NIT states that the candidates should have passed their 10+2 level with atleast 75% aggregate, and from a recognised board. This means you need to score atleast 75% aggregate overall, and not only in PCM. Since you have scored 82% in ISC, you are eligible for NIT. Prepare well for your JEE Main and try to get atleast 98 percentile to get a seat in the top NITs.
Hope this answer helps you.
Good luck!!