i am of obc catagery should i get nit or iiit in 65% marks in board exam

No. You need to have at least 75% aggregate in 10+2. Only then you are eligible.

Hii satyam,

If you are obc then too you aren't eligible for admission in any of the two.
It is clearly written in the information brochure released with the form that
"Candidates appearing for JEE Main 2018 must
have passed class 12th with Physics, Chemistry & Mathematics in order to apply. However For admission to IITs, NITs, IIITs & CFTIs candidates must have secured at least 75% in their class 12th board exams (65% for SC/ST students)."

These lines are  clearly written in it. There is no chance of any kind of modification.

Sorry, but don't get disheartened.
There is always a second chance.

Good luck.