NIT

It depend, in which stream you want to take admission. But, approximately you need 170 marks for top NITs.

By preparing and of course scoring well in JEE main, you can get admission in NITs, but to get admission in IIT you have to first score well in JEE main and required 75% marks in class 12th. Then you are eligible for appearing in JEE advance examination. If you score well in this exam, you can get admission for IIT. To get admission in IIT, you have to do smart work and it's totally different from the preparation of JEE main exam.

Yes, you can. To get admission at NIT, you have to pass JEE entrance exam with good score.

It depends on the normalized score, which will predict your rank.

For any reserved caste, the cut off is 150 marks and for others 200+ is required for best NIT.

JoSAA counselling is closed.

For mechanical engineering
NIT, Surat; NIT, Durgapur; NIT, Warangal; NIT, Surathkal; NIT, Srinagar, and MANIT are the best options.

It depends on your home state quota and your branch preference. It would be better if you can add some more details to your question.
Thank you.

For OBC category, 180+ would be safe for CS branch. Also, it depends on your state, whether state quota or all India quota. But, top NITs are NIT Warangal, Surathkal, Srinaga and Durgapur.
All the best.