Shucking oysters takes a little practice, and there are a couple of approaches to try, but soon enough you'll find your rhythm. Here's what works best for me:
Brush the shells clean with a stiff brush under running water. This gets rid of a lot of the grit.
Place the oyster, cup-side down, on a clean dish towel on the work surface and fold the towel over it. This not only gives you a stable platform for shucking, but the top fold will help protect your hand should the knife slip.
Probe with the tip of the oyster knife to find the gap in the hinge at the back of the shell. You don't have to use much force.
Slide the knife into the gap and give it a twist to pop the hinge. The hard part is done.
Run the blade along the top shell, severing it from the oyster. Work carefully here, because you don't want to spill any of the oyster liquor.
Hold the cup side and cut underneath the oyster to free it completely.