First of all I would want you to know thoroughly about the Merchant Navy career which can be seen from the following link-

https://www.careers360.com/careers/merchant-navy

After having a clear picture now, you will have an idea that merchant navy requires a lot of interest , mental fitness and physical fitness as well.

Now for qualification you need to have physics, chemistry and mathematics in class 12. The passing percentage must be 60%. If you are fit in this category then you can give relevant exams in this filed and get admitted in any of the marine engineering colleges for study of courses like- BTech marine, DNS, BSc Nautical Science etc. This list of exams can also be found in the above link.