Hi Dear,

If you want to join merchant navy after 12th, then you will have to write Indian Maritime University Common Entrance Test (IMU CET)  exam.  Depending on the marks you get in IMU CET, you'll be accepted into the various campuses of IMU and its affiliated institutions.

You can check the following link for complete details:

https://engineering.careers360.com/articles/imu-cet

Good Luck