Clean the clams under cold running water.
Then open them, either by holding the clam in your hand in a clean towel or putting it on a chopping board, and running a knife between the shells (always moving the knife away from you).
If you can't manage to open the clams this way, put the clams in the oven or grill/broil them for a minute or two, which will make the shells open up.
Keep the juice which runs from the clams as you open them - your recipe may require it and in any case, it's delicious.
When you're buying your clams, allow six per person, or four if they're large.