Hii!
All colleges have different process to enroll students into NCC. The process is not tough though and you don't need to worry. The wing, in which you apply to, that is, if it Army, Navy or Air force, also makes the process a little different. But the basic things are, physical fitness in terms of endurance, flexibility, class 12th percentage may be taken into consideration, a written exam on basic knowledge of armed forces and general knowledge. So, be ready with all of it and you will be able to become an NCC cadet soon.
Hope this helps.
Good luck!