Hold the oyster in a towel or oven mitt with the hinge (pointed side) sticking out. This will protect your hand if the oyster knife slips.
With the flat side of the oyster up, insert the tip of an oyster knife near the hinge. You only need to insert the knife about 1/2 an inch. Do not use a regular knife to open oysters. There is too great a risk of slipping and severely injuring yourself. An oyster knife is a special blunted knife made for this purpose. They're cheap, too. If you're going to buy oysters in the shell, spend a couple extra bucks for the proper knife.