You haven't mentioned the exam name which you are asking about. So it is quite difficult to tell you the exact eligibility criteria to be eligible for BSc Nursing as the criteria is different for different colleges.

However, as per the eligibility criteria of Indian Nursing Council (INC), the student must be at least 17 years old as on December 31st of the year of admission. And this age criteria is followed by almost all the nursing colleges.
The candidate must be medically fit.
As per INC, the candidate must has passed 10+2 with PCB and English as core subject with at least 45% marks from a recognized board. But the minimum marks criteria is sometimes different in different colleges.

http://www.indiannursingcouncil.org/nursing-programs.asp?show=elig-crit