The only way to join NIT colleges in our country is by giving engineering entrance exams conducted by government. Inder this category all IIT institute and NIT institute comes and some other govt. affiliated colleges.
To get an admission in IIT you must get a rank of under 10000 and to get a good NIT college, you have to get a rank below 40000 so that you can get a good NIT college. You can also get NIT college till 60000 also but you have to wait till for 2nd and 3th counselling.
It is very difficult to get nit after 60000 as almost all seats are filled, and if you get you would not be able to get your choiced branch.
So hardwork and get good rank for nit