Merchant Navy is a very generalised term and there are different departments and different ranks. I am answering to this question assuming you want to join as sailing staff.Firstly you have to decide which department you want to join.For joining as an Officer.Deck Cadet and Engine Cadet also known as Trainee Navigating Officer and Trainee Marine Engineer respectively you have to appear for IMU-CET (Indian Maritime University Common Entrance Test) held twice a year. Apart from this you will also have to secure a sponsorship with a company in order to engage in onboard training upon completion of the course for which you will have to appear Company specific tests, interview and medical examination, individually for each company you are applying. If you clear these two then you will be selected to undergo training course.If you want to join as Trainee Electrical Officer you need to apply to company directly for the pre-seatraining course.For more info regarding eligibility criteria, procedures and general info you can checkout other answers in my profile.For joining as a crew/rating :Appear for entrance exam conducted by reputed institutes offering 6-months G.P Ratings course.Upon completion of course you will be placed onboard for training by the companies coming for campus placement or by your own efforts and later promoted based on performance.Request an edit if you want more specific answers on any part of this answer.