First of all, the military is not a job rather its a service to the Nation. Now as there is nothing specific as a central job in military assuming you are asking for an officer or High-rank posts in Indian armed forces, then You need to appear for NDA after 12th Or CDS after your graduation. If you want to join as an Engineer then there are some Departments of Engineering which are offered Service. Electronics and related, Mechanical, Civil are some of the departments which have an advantage over others. Preferably if you are doing any Engineering or Science courses then chances of you are more, to join as an officer because many posts need good mathematics.