Hi,
To join indian army, you should go e NDA exam after 10+2 for which minimum eligibility are:-

• 10+2 from any stream
• Candidate must be unmarried
• Candidate must be fit
• Candidate must be between 16.5 years of age to 19 years.

For NDA, you must know that it is conducted twice a year and there are 3 parts of it:-

• Written
• Interview
• Medical test

For more information check the NDA section of official site,
http://www.upsc.gov.in