Most cruise line internet sites have, amongst other parts of their menu, a bit that says either "Jobs" or "Work for us". Just click on this and information about working for the company will be displayed. Of course there are many ways of working on a cruise ship. You could be a hairdresser, beauty therapist, photographer, wine waiter, entertainer or shop assistant. Experience in such a field would be a good start! Many of these people work for the agencies that the cruise company has a contract with. So contacting the agencies is also a good way to start your search for cruise ship jobs. Good luck.
Type cruise jobs into Google. It's as easy as that.