Recruitment in the defense forces differs for different levels.
1. Recruitment at soldier level.
This is done via recruitment drives and recruitment centers that are present at a number of locations in the country. They send out the advertisement for the dates of next recruitment drive.
For entry at this level, basic requirement is matriculation with 45%. For entry of technical posts, minimum eligibility criteria differs depending on the field.
2. Recruitment at officer level.
This is done through national level competitive exams or through direct selection for SSB in some cases.
After passing class 12th, one can give exam for NDA. Merit in this exam allows entry into Army (non technical), Airforce, Navy (technical + non technical). This will result in admission into National Defense Academy, Khadakwasla (Pune) or Indian Naval Academy (Navy technical).
For Army technical, one can apply for TES (technical entry scheme) on the basis of marks obtained in 12th boards. This will result in admission into Officers Training Academy, Gaya.
Alternatively, students having NCC 'C' certificate can directly apply without appearing in an exam.
After graduation, one can give CDS exam for entry into Army (technical+non-technical), Navy and Airforce. In case of Airforce, it is essential to have physics and mathematics as subjects during graduation. In order to enter the technical stream(of any arm) it is essential to have an engineering degree during graduation. Clearing these exams will result in your induction into Indian Military Academy, Dehradun.
Another option after graduation is to apply for short service commission directly. If selected, it will lead to induction in Officers Training Academy, Chennai.