Hii!
To join Army, you can directly apply after class 12th in NDA entrance exam, if you pass class 12th with science stream or specially physics and mathematics.
Also, you can apply for the Army entry, after you complete graduate, either via the physical selection procedure held at different times in different states or via entrance exams such as SSB, CDSE, UPSC, etc.,
You need to have a minimum of 50% marks in graduation from any relevant subject discipline from a recognised university for applying after graduation.
Hope this helps.
Good luck!