NIT

You scored well in JEE Mains, in order to get admission in NIITs.

Dear Karan,
Although after IITs, only BITS Pilani is considered as a good private engineering college. For this, you need to score good score in BITSAT exam.

IIT JAM, 2018 Subject-wise Cut off is mentioned below. Please check, if you made the cut from the marks mentioned below. In Physics, qualifying marks for general category are 19.51 while for SC/ ST/ PWD, it is 9.76. For Physics Paper/
In Chemistry, qualifying marks for General Category students are 14.79 and 7.39 for SC/ST/PWD. For Chemistry Paper of IIT JAM 2018. In Mathematics, qualifying marks for General Category are 23.35 and 11.68 for SC/ST/PWD. Based on IIT JAM Result 2018, candidates will be admitted to Institutes according to the preference of choices filled by them in Admission Form. Regards.

You must get 75% marks in class 12th for getting admission in NIT 2018.

No, you can not join NITs after clearing NERIST
The NERIST Entrance Examination (NEE) is conducted by North Eastern Regional Institute of Science And Technology NERIST for admissions to various B.Tech programmes, offered by NERIST. The entrance examination is conducted at different centres in the State of Arunachal Pradesh, Assam, Meghalaya, Manipur, Mizoram, Nagaland, Sikkim, and Tripura. For more details check the link below:
https://www.shiksha.com/university/north-eastern-regional-institute-of-science-and-technology/exams/nerist-entrance-exam.

There aredifferent cut-offs for different categories but if your score is above 150, you will definitely get a good college.

You need to take minimum 75 marks in JEE Mains to eligible for NIT Colleges.

Yes, you can get. Contact the respective colleges ASAP.