To get into Indian navy, there are mainly two ways. Either through NDA or CDS
To get from NDA, you need to pass 10+2 with physics, chemistry and mathematics. (English is compulsory)
To get from CDS, you need to be science graduate with subjects like physics, chemistry and math’s. (English is compulsory)
For both the above, you have to sit for examination which is conducted by UPSC (union public service commission), who advertise for this particular examination once a year for both.
For CDS Maximum age is 23 and for NDA it is 19 years.
Passing NDA you have to go under training and also have to pass graduation exam affiliated to Jawaharlal Nehru University, New Delhi.
And after passing CDS, you only have to go under training for nearly one year and will get into force directly.