If pre-opened, they can't re-seal their shells.

They should be closed when you buy them (touch it and watch it close - this proves it's still alive) Rinse any debris off of them, put them on a cookie sheet and put them in the oven at 350, heat until they open. This should not take but a few minutes so don't walk away from the oven. (If grilling, put them on the grill - same outcome.)

If you're opening by hand (no heat), put the blunt point of your oyster knife at the point where the shells hinge together. It'll take you a few minutes to figure out the right place to put the knife and twist 'just right' to open it.

If you want to watch a pro do it, ask the person working at an oyster bar/raw bar to show you how. It'll be worth buying an order to watch him/her open them. It's not hard at all once you figure out the right place to put that knife.

Word to the wise - you should wear some kind of garden glove because the shell IS sharp and your initial efforts with the dull-bladed oyster knife might slip and cause injury. Some will use a flat-blade screwdriver for this purpose - use your best judgement - some oyster knives have a shield that will protect your knuckles.