For getting into Army after 12th one of the way is to crack the UPSC-NDA exam that is being conducted by UPSC (Union public selection board) twice every year named as NDA & NA-I and NDA & NA-II. For details you can check UPSC website, its been listed under the section NDA and NA exam.
You can apply for NDA exam between 16.5 and 19.5 years of age.
Other way is TECHNICAL ENTRY SCHEME ( 10+2 ) Or TES (10+2)
Age requirement is 16 to 19 yrs as on first day of the month in which course is due to commence.
You can join also INDIAN ARMY through Zone wise rallies which is conducted in Every year in all over the India for JCO or Soldier.
Hope this helps.