First of all, you need to appear select a course according to your qualifications and then appear for IMU-CET examinations. Once you complete your pre-sea training, you will be placed as a deck officer.

Gradually, when you start gaining experience, you will be promoted to higher ranks on the ship. After crossing through multiple designations, clearing examinations, and obtaining years of experience, you can be promoted as the captain of the ship!

For example, if you opt for the GP Rating course, your career path will look something like this:

Join as the trainee > get promoted as Able body seaman > Get promoted as Bosun > Get promoted as deck officer > get promoted as 2nd officer > get promoted as Chief officer > Become the captain of the ship

DNS Course is the best course for being a captain in the merchant navy.Join good colleges such as anglo eastern , great eastern , samundra institute , tolani institute.etc

You need to appear the entrance exam like IMU CET conducted by Indian Maritime University, CUSAT CAT etc. for Merchant Navy. You need to become first a Deck Officer & continue to gain experience and receive on-the-job training, deck officers may qualify for promotion to captain. Years of experience required to reach this point. You can check the eligibility of the exams with given below links.

IMU CET Eligibility

CUSAT CAT Eligibility

Top institute for Merchant Navy you can prefer:

Marine Engineering and Research Institute, MERI Kolkata
Marine Engineering and Research Institute, MERI Mumbai
Lal Bahadur Shastri College of Advanced Maritime Studies and Research, Mumbai
T.S. Chanakya, Mumbai
National Maritime Academy, Chennai
Indian Institute of Port Management, Kolkata

Good Luck!