Hello Akshay,
Your Marks in 10+2 is only considered for eligibility criteria. For Admission in NITs, You have to appear in JEE Mains. Your good rank in JEE Mains can help you for admission in NITs, Not your percentage in 10+2.
For other Government college, Like IITs, you have to appear in IIT JEE Advanced after JEE Mains.
For State Government college and Government University, You have to appear in State Engineering Entrance Examination.
Good Luck.