See Candidate firstly you have to be appear in SSB interview conducted by the UPSC. If you will qualify SSB interview then your medical will be done. If you found Fit in all aspects then preparation of Merit list Will be there. Hence if you getting more marks and securing Good rank them you will be getting any of the defense forces of your choice. If not then you have to be satisfy yourself the Branche which you are getting.Here is the link in which Complete details about NDA SSB interview is given. https://competition.careers360.com/articles/nda-selection-process

Hope you understand this very Well and All the Best..