Mussels should not be intentionally opened before they are cooked. They open up in the cooking process. Cook mussels by steaming them in a pot covered with a lid. They are done when steam escapes from the lid and they have opened up.
If the mussels are open before the cooking process, rinse them with fresh water and they should close on their own. Tap the shell if they don't close on their own to force them closed before cooking them. Toss any mussels that don't close. Don't completely cover the mussels in water and boil them, because they won't cook properly and won't open when done.
Use water, wine, beer or other liquids to steam the mussels. A variety of aromatics can be added for additional flavor.