NIT

Hello Balu,
Since the cut-off for ST category students for JEE Mains 2017 is 27, you have passed it but the score is not very good. You may use the college predictor for JEE Mains which may suggest some colleges for you:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor
But, I think, you may not get a top NIT and have medium chances of getting an NIT.
Mention your ranks also so that I may help you.

Hello Balu,
As per your marks and your category your chances are nil for getting any nit's,however for more insights refer to this link:
https://www.shiksha.com/b-tech/resources/jee-mains-college-predictor

Your chances are too low because you need at least 75% in 12th to get admission in NITs.

Yes, obviously, you have to score 75% in boards to be eligible to get admission in any NITs.

IIT Roorkee offers admission on the basis of AAT (Architecture Aptitude Test).
AAT is an architecture entrance exam for admission into IITs. Moreover, for appearing in AAT you have to clear JEE Advanced also. To appear in JEE Advanced you should have cleared JEE Mains paper 1.

There are chances for you to get NIT as you are from reserved category but your state should have NIT with architecture branch.
Hope it helps.
All the very best.

Can you tell your mains paper-2 rank and your category since fir B.Arch, you have to fight paper.

I dont think all NIT's have this provision but yes there is a provision of branch change after 1st year though the requisites for the same may differ
Like in NIT Kurukshetra you have this provision. You can apply for branch change provided you dont have any backlogs in the first year and leaving your current branch doesnt let the strength fall below 75% of the total strength(including the vacancies) present in your current branch. And cgpa is not an issue if you fulfill the aforesaid conditions.