I want to know about procedure to join merchant navy after passing 12th grade with PCM subjects ?

Yes you can join Merchant Navy after passing 12th grade examination.. to join merchant navy you have to pass one of the following courses .

3 month Pre-Sea Course in Deck rating.
3 month Pre-Sea Course in Engine rating.
4 month Pre-Sea Course in Saloon rating.
STCW Course
1 year Pre-Sea Diploma in Nautical Science
HND (Higher National Diploma) Nautical Science
B.E Marine Engineering (4-Year Degree course in Marine Engineering)

Eligibility
You have to completed 12th std. with Physics m, chemistry and mathematics from Recognized board of India.

Age limit:
Age of Candidate should not exist 21 years

Physical requirements:
Eye Sight = 6/6,
Height = 167 cms,
Weight = 50 Kg
Chest = 77 cm (Normal) / 82 cm (Expanding)

The selection in Merchant Navy will be on the basis of following:
Written Examination
Medical Examination