Dear Rohit,

In order to take admission in IITs/NITs/IIITs and GFTIs you should have 75% marks in your board exam or you should be in top 20 percentile. 

To know about the top 20 percentile and how it is calculated you can refer below link

For IITs

https://engineering.careers360.com/articles/top-20-percentile-cutoff-of-class-12-boards-for-iit-admission

For NITs/IIITs and GFTIs

https://engineering.careers360.com/articles/top-20-percentile-of-class-12-boards-for-nits-iiits-and-cftis

Good Luck!!