How do you cook mussels properly?!

before cooking them, pick out any oysters that are open and don't close when gently squeezed. those are dead, and thus not good to eat. pull of any "beards" (the nasty, fibrous pieces hanging near the hinge of te shell). depending on how you're preparing them, you may need to shuck them. to shuck them, insert a heavy, dull butter knife or shucking knife into the side of the oyster and twist (youtube has a video that shows it in detail). you may want to hold the oyster in a towel in case the knife slips- that could be bloody. if steaming, don't shuck them. place them in a steamer rack then insert into a pot that has water (and any aromatics/spices you desire) filled to just below the steamer rack. Steam until the oysters open (6-9 minutes roughly). you can also make fried oysters, oyster po' boys, oysters rockefeller, oysters on the half shell... the list goes on.

Remove the 'beards' and scrub well
In a dutch oven place the mussels inside and cover with broth or stock and some white wine.
If you have some sprigs of parsley or a couple of bay leaves throw them in too.
Cover and steam over medium heat for 10-15 minutes till they open up
Serve with lemon garlic butter and french bread to dip into the juice.

steamed fried etc help me by adding detail and i will improve my answer sweaty