Anyone with an 8th, 10th, or 12th grade, as well as a graduate or postgraduate degree, can apply for a defence job in India. It is now up to you to decide which defence line job you want and to prepare accordingly. Also, candidates should  between 18 to 21 years of age to apply for defence jobs in India.
Some of the top defence jobs include:-

Indian Army
Indian Air Force
Indian Navy
Indian Coast Guard
CISF
BSF
CRPF
SSB

Check out this link for further information:

https://www.india.gov.in/spotlight/career-defence-and-paramilitary-forces