Warning: If you are looking for a leg up on these clams, ice them first as this causes the muscles to relax, and never handle the clams roughly before trying to open them or it will be an exercise in frustration.
You will first need a clam knife. Any short thin blade knife will do. A round tip helps. The Dexter Russell S127 is the industry standard. Many people successfully use Chinese knockoffs, such as the MortyTheKnifeman.com 522-P.
There are two popular ways of opening clams with a knife. It is best to be shown by someone who is good at it.
The proper way in: Hold the knife in your dominant hand, and in your other hand position the clam with its lip facing out towards your fingers and the hinge facing in toward the base of your thumb. The clams fatter, rounder end should be down towards your pinky finger. Apply the knife edge carefully in the groove between the two lips, and use your bottom two fingers to apply steady pressure to the back of the knife. Do not try to use your knife hand; this increases the chance of slipping. Simply squeeze the knife in and use it to cut the two muscles holding the clam closed. If you are careful you can run the knife along the roof of the clam and sever these muscles without cutting the meat. Next simply cut the other ends of these muscles. It takes some practice to open clams without mangling the meats.
Some folks like to go in the "backdoor". Most will only resort to the back door when the proper way in doesn't work. Apply the knife edge to the hinge in the back and work it until you break or loosen the hinge, or use a mallet to drive the knife through the hinge. Then you can turn the clam around and proceed in the normal fashion. This way in increases the risk of breaking bits of shell loose into the meat.
There are two ways to cheat the shell using temperature. A preheated oven at 450 degrees for 5 minutes or a little dose of microwave oven works as well. Clams also relax their muscles if placed in ice water or the freezer for an hour. Littlenecks to chowders can be frozen solid, then run under tepid water until the shells gape just a little. Then take a table knife and pry apart, scraping the top shell membrane down. For chowders, chop them into 6 to 8 pieces and put into a stainless steel bowl to defrost. Add to recipe after defrosting... extra liqour can be added if flavor is not strong enough. If doing a baked clam, let defrost on a sheet pan and then add toppings to bake.
Adapted From Wikipedia, the free encyclopedia