What should I do after 12th for defence
It's eligibility criteria says the candidate must be unmarried and below 18 to apply for its papers.
You can get all the necessary details about the exam on the given website.
https://www.sarvgyan.com/articles/nda-2018
where cadets of 3 sections of the Armed Forces- Army, Navy and Air Force train together for 3 years and are well groomed for Defence roles.
Students have to appear for nDA exam post 12th and cleae SSB Interview to join such cojrses.
Entire process explained below.
https://www.apnaahangout.com/join-indian-army-officer-12th-science/