Dear Shubhi, after 12th, you can join the Navy either under Sailor Entry or Officer Entry.

Under Sailor Entry for:

• Artificer Apprentice, the minimum educational qualification should be 10+2 with subjects Physics, Chemistry and Mathematics as mandatory. Candidates should be aged between 17-20. 
• Senior Secondary Recruits: the minimum educational qualification should be 10+2 with Physics and Mathematics as mandatory and Computers, Chemistry and Biology as optionals.

Under Officer Entry: The types of entry under this category are Executive, Electrical and Engineering.

• The mimimum educational qualification is 10+2 with subjects Physics, Mathematics and Chemistry as mandatory and aged between 17- 19&1/2
• NDA/Naval Accademy: the minimum educational qualification is 10+2 with subjects as Physics, Chemistry and Mathematics. Candidates appearing for 10+2 are also eligible. Candidates should be aged between 16&1/2 and 19.

You can give NDA (National Defence Academy) Exam which takes place twice in a year in the month of April and September. 

Age criteria is that the candidate must lie between 16.5 to 19.5 years. 

For Indian Navy :- The candidate must have passed/ appearing 10+2 from a recognised board or university with Physics and Mathematics as compulsory subjects. 

Candidate must also be physically and mentally fit. 

This is one of the ways you can join Indian Navy there are other exams too. 

If you still have any queries feel free to ask in the comment section down below.