You have to appear for jee mains engineering entrance exam in order to get admission in NITs.You need to have a minimum marks of 140 around jee mains to get admission in NITs with some branches.
JEE Main exam is conducted for admission to NITs. JEE Main 2019 exam will be conducted by NTA and exam will be held twice a year.
Refer to the link below for detailed information about the entrance exam
Hope this helps.