There are 2 entries after 12th. One is NDA and the other is TES. For NDA there will be a written test first and for TES Navy there will be direct interview on the basis of 12th cutoff.

NDA eligibility:

The educational qualification: you should be passed 10+2 (with physics and mathematics) to apply for the exam. The exam is conducted twice every year

Age limit: Your age limit should be 16 to 19 Years.

For further updated information you can go through with given below link.

NDA

TES Eligibility

Education qualification: You should be passed 10+2 Examination or its equivalent with a minimum aggregate of 70% marks in Physics, Chemistry and Mathematics from recognized education boards.

Age limit: You must not be below 16½ years and above 19½ years.

Good Luck!

NDA is a national level entrance exam conducted by Union Public Service Commission (UPSC) for 10+2/ HSC candidates who wish to pursue their career as commissioned officers in Army, Navy, Air Force and Indian Naval Academy Course (INAC). NDA/ NA 2017 is conducted twice a year. For further detail about you can go through with given below link.

NDA 2017