To open, use a short, stiff-bladed knife. 
Set clam upright with hinge side in left hand.
Insert blade about one inch up from the bottom, and cut in and down. (You are cutting through one of the muscles that holds the clam closed.)
Turn clam over and repeat process from other end.
Pull shell apart and wash thoroughly.
Lay on side and cut mean from shell on both sides.
Pull off the trimmings.
The dark portion is the liver, which is good to eat, as is all of the clam.
Update: Only the white meat of the clam and the muscles that hold the clam shut are safe to eat. Any digestive portions and dark meat should be discarded.
Also, the muscles that you cut that held the clam shut are good to eat with cocktail sauce.