There are 3 ways in which you can enter the army.
1) The first option is the NDA process. The education qualification required for this is 12th standard. You must have pursued Physics, Chemistry and Mathematics. Also there is a maximum age limit of 19 years. You will need to qualify in the NDA entrance exam.
2) The second option is to join after graduation. You will need to clear the CDS exam.
3) The third option is University Entry Scheme where the candidates from Engineering back ground are allowed to apply. The maximum age limit is 25 years. You must be on the lookout for notifications regarding this.