Firstly, you need Physics, Mathematics and Chemistry from any board or school for class 12.

You need to have a good vision. Medical exams are also a must, for screening of your fitness.

And, after class 12, you will have to clear an entrance exam (Indian Maritime University Common Entrance Test)

and subsequently,  there are a number of private universities providing the course. They train you to become deck cadets etc.
Some courses available to you are Bsc Nautical Science, BSc Marine Engineering, Bsc Petroleum Engineering, BE civil engineering.

If you want to go directly after class 10th, you will have to involve in courses like Near Coastal Voyage, Diploma in Nautical Science, Commerical Diving, GP Rating etc.