Many people seem to be intimidated by the idea of cleaning and cooking mussels, but it is really very simple.
First, tap the shells discard any that don't close. They must be alive when you cook them, and if they are alive, they will defensively close up when tapped. Just before cooking (not in advance), rinse the mussels under cold running water and pull or cut any visible "beards"–those hairy strings that hang out of the shell. That's it!