Hii!
Actually, you can join army in so many ways. One of the most common way is through UPSC, via SSB, CDSE, ITBP, CRPF, BSF etc.,  You can also apply for AFCET, etc., All these exams, after you clear them, will help you earn a officers rank in army. But, you have to complete your graduation first.
Hope this helps.
Good luck!