First and foremost, taking a screenshot is usually bound to your "Print Screen" key -- whenever you're playing the game, just hit that key on your keyboard, and a screenshot (in JPG format -- it used to be saved in TGA, but Blizzard changed that a while ago) will show up in the /Screenshots folder inside the World of Warcraft installation on your hard drive. Of course, on a Mac keyboard, there is no Print Screen button, so I have mine bound to the "Home" button instead -- you can rebind it to whatever you want in the options menu. And that's just the basics -- it gets more complicated from there.
First thing you'll probably want to do is pretty the screenshot up by getting all of those messy UI elements out of the way -- you can do that by pressing Alt-Z before you take the shot. If you want to zoom way back and get a big wide shot, you can either change your camera distance in the UI, or do a little code tweak to make it even wider. You can change your video mode, too, if you want to get the best graphics you can.
And finally, addons can help as well. Multishot will automatically take screenshots for you at opportune moments -- leveling, boss kills, and so on. PhotoOp will do the same thing, and even has a script API that will let you script screenshots whenever you want. Deathsnap will hit the button when you die (in case you want to see what killed you?), and FreezeFrame seems really interesting -- it has all sorts of extra options to add to your images, including putting a stamp at the bottom of which character you're on, and where you were when the shot was taken.
As for making your screenshots look better, you might even want to turn to traditional photo composition tips for that one. But the addons and UI tips above should have you doing as much screenshotting as you want in no time.