Cultivated mussels are quite clean and do not require much prep work - simply rinse mussels and remove the "beard" if still attached. Mussels may open slightly when exposed to air. To test mussels for freshness, tap the shells of any that are open. If the mussel's shell does not close, discard it and do not cook it. As well, fresh mussels will sound solid, not hollow, when tapped with a knife.
Place mussels in a pot with 1/2 cup water or white wine. Cover pot, place on stove and steam over moderate heat for 5-10 minutes until shells open wide and meat loosens from shell. Discard any mussels that do not open. Serve with melted butter for dipping - some like to mix melted butter and vinegar together for dipping.
For reference - one pound of mussels will yield about one cup of mussel meats. For an appetizer allow 10-12 mussels per person, for a main course allow 20-25 per person (or more).