Hello,
For students from General Category, one must have an aggregate of at least 75 percentage in their CBSE Class 12th boards or one needs to be in top 20 percentile of their respective boards to get admission in IITs, NITs and IIITs. For students from SC/ST category, one must have an aggregate of at least 65 percentage in CBSE Class 12th boards examination.
Admission in NITs and IIITs are based on JEE Mains rank and admission in IITs are based on JEE Advanced rank.
I hope this helps. For further queries, ask me in the comments.