If you want to go into defence services as an officer just after class 12th then the only option is to appear for National defence academy exam and qualify the SSB interviewthe qualification is you need to have physics as your subject in class 12th and mathematics if you are applying for airforce and navy.

Other option is technical entry scheme in which you need to have physics chemistry and mathematics for subject in class 12th.

if you wish to join defence forces as an officer after your graduation then there are multiple entry option technical entry , general entry by cdse.