Hello dear..

You need to compete in JEE Mains for getting admission into NSIT Delhi. After that you are required to apply for counseling in jacdelhi.nic.in for admission.
I am considering that you are from Outside Delhi General category, so you need to have a good AIR at least 4-5k for getting a decent branch like ECE,ME,etc. For getting COE or IT you need to have an AIR less than 2.5k.
P.S. My information is based on last year's​ cutoff.

Best of luck