You can join in Merchant Navy by below following courses after 12th

B.Sc. Nautical Science
B.E. Marine Engineering
B.E. Naval Architecture and Offshore Engineering
B.E. Petroleum Engineering
B.E. Mechanical Engineering
B.E. Harbour & Ocean Engineering
B.E. Civil Engineering
B.E. Electrical & Electronics Engineering
B.Sc. Marine Catering
Electro Technical Officer Course

After 10th

Near Coastal Voyage (NCV)
Diploma in Nautical Science (DNS)
Commercial Diving
G.P. Rating