My dear navneet,
You said that you are 21 years old and you wish to join army. For your kind information, The Staff Selection Commission and Union Public Service Commission conducts examinations yearly for this types of recruitment. The Organization of army also conducts the recruitment rally twice in a year. The eligibility for each examination is different. If you wish to attend the recruitment rally, you should have passed higher secondary (10+2) with any stream and your age should have between 17 1/2 years to 22 1/2 years. The first step in this rally is physical entrance test in which you will be completed 1600 meter run test in 6 minute, 20 seconds. If you have passed in physical test, you will be called for written test and then medical check-up. You should practice at least 3 months for running test and for making good stamina. You should set your timing at the time of running so that you can make improvement daily. You can do this in 45 days if you hard work. Not to worry about your age because you can attend its next time till you will be 23 years old. Get some boiled chickpeas with glass of pure milk in the break fast for making the strong stamina. Avoid junk food, smoking and drinking (if you have its) till you will be got it. You can try SSC and UPSC after your graduation also till the age of 28 for Officer entry. So keep it up and make future as a soldier.
Be a men,