The main reason for shucking oysters is to eat them raw. You want very fresh, live, fabulous oysters if that's your plan. Luckily, oysters are fairly cooperative as long as you keep them cold and don't suffocate them in plastic. I like to buy oysters the day I'm going to shuck them, but that's mainly a storage issue. There is no reason not to buy them a day or two ahead if that's more convenient.
Above are a few Atlantic oysters (Crassostrea virginicas). I chose them because, in my humble opinion, they are trickier to shuck than their Pacific counterparts. Even so, you'll see in the photos that follow how easy they can be to open if you know what you're doing and don't try and rush yourself.
Along with the oysters, you will also need a short knife or other thin-edged instrument. I use an actual oyster knife in these pictures, which is nice because it has a guard around the blade to keep your hand from slipping, but I've shucked plenty of oysters with a screwdriver (flat, not Philips, head!) and even a table knife. You want something with a thin edge that you can work between the shells but that is also strong enough to use to pry open the shells. Most people will also want something with which to hold the oyster (those shells can get sharp). A rag or kitchen towel or oven mitt are all good options, just note that whatever you use will get pretty darn acquainted with oyster shell and oyster juices and may never truly be the same.