If you're really interested but don't want to break the bank, check craigslist in your area for a used (crack free) pre-formed pond. They can be picked up for almost a song. Sometimes you can find pondliners on sale too, just make sure they're UV resistant and MADE for ponds. If your yard is small, a 100 gallon pond only covers about 3x5 feet and is JUST deep enough to keep a couple of fish in. If you go the pond liner route, you can make it deep, with a slightly smaller footprint, but 100 gallons is really the best starting point if you plan to keep fish. Usually, the craigslist people will include the pumps you'll need for water circulation and building a DIY filter is about as easy as DIY's go (5 gallon bucket, pump, filter media, drill for the lines... done). Post pics on what you decide!