After completion of 12th class, interested candidates can appear for the Indian Maritime University Common Entrance Test (IMU CET) or the All India Merchant Navy Entrance Exam.
The eligibility for joining merchant navy is finishing class 12 with Physics, Chemistry and Mathematics.
Candidates have to be unmarried Indian citizens (male or female).
Normal vision is required but glasses up to plus or minus 2.5 may be allowed.
Admission is through an entrance followed by a screening test and a main written examination.
After clearing the test, there is an interview and a medical test.
You also need to complete a ship-training course prior to the employment. The course is generally short-term where the candidates are taught basic security concerns of voyaging.
I hope this helps.