iam an st student. I got 77 percentile in jee mians. Can i secure a seat in any of the nit? if so how the college would be?

According to your percentile, your predicted CRL rank will be around 2,45,000 and category rank will be in the range 4300-4500. Unfortunately you have very low chances of getting any NIT or IIIT. You might get some lower ones in the special round like;
1. NIT Srinagar ( ECE, IT )
2. IIIT Kancheepuram
3. IIIT Pune ( ECE )
4. IIIT Dharwad ( CSE )
5. IIIT Guwahati ( ECE )
6. IIIT SriCity ( CSE )
7. IIIT Ranchi ( CSE )
8. NIT Agartala
9. NIT Rourkela ( Food Processing )

Check out the link below to explore college options according to your percentile.

https://engineering.careers360.com/jee-main-college-predictor

Good Luck !