NDA is for unmarried male and female candidates in the age group given in the UPSC notification (roughly 16.5 to 19.5 years). For the Army wing, Class 12 in any stream is enough; for the Air Force and Navy wings, Physics, Chemistry and Maths in Class 12 are required. Selection is through a written exam followed by the SSB interview. Check the official UPSC NDA notification for the exact criteria each year.