Contrary to ChipB's suggestion, I would strongly recommend
avoiding the use of a circular polarizer for outdoor photography, because once you see what it can do you'll never want to shoot outdoors without one again!
Seriously, though, if you do go for a polarizer or any other filter, spend the extra and get a better filter - you definitely get what you pay for. Especially if you have a better lens. (Who would want to put bargain tires on a Mercedes?) Anyhow I'm not talking about a Singh-Ray brand of filter if you're just starting out, but a Hoya or B+W filter would be a great balance between price and quality.
Some people also use the trick of buying a filter with a 77mm diameter for their lenses with 49 or 58mm filter diameters and using step up rings to allow the filter to fit their lenses. This way, if they ever buy a wide angle or other lens that requires a 77mm filter, they already have one and don't have to buy more than one type of filter. So there's my 3 cents' worth.