If you want to get into defence services as an officer then there to is I let you go after class 12th or after your graduation.

after class 12 you can appear for NDA and after qualifying NDA you need to appear for SSB interview if you make into the merit list and then you will be joining National defence academy khadakwasla Pune after 3 years training you will be sent to specific arm of the defence force that is if you choose Indian airforce then you will be sent for training at Hyderabad and respectively Dehradun four military academy and for civil services will be sent to respective academy after training for around one and half year you will get commissioned into army. Service equivalent to group A gazetted officer.

if you appear for CDS after graduation then after qualifying CDS you need to prepare for SSB and if you make it to the merit list then based on the preference you will be sent to the respective academy of the army of the defence forces. The pattern is similar after training for around one and half year you will get commissioned into the defence force.