greetings,
to become a first class officer in the Indian Army (non technical, medical or legal posts) you will have to clear the NDA exam (after 12th) which has an age limit, otherwise the next best shot is the CDS exam (after graduation).
for more information on these exams, refer to these links:
NDA - https://competition.careers360.com/exams/nda
CDS - https://competition.careers360.com/articles/cds-exam
good day
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