Swordtails are beautiful, hardy fish. Like all livebearers, they can overpopulate a tank if not kept in check, but they are not as prolific as guppies. Dominant males are easily identified by their long "swords" at the base of their tail fins. These fish are best kept in trios of one male to at least 2 females. Males can become aggressive with each other, competing for mates, and I personally would not keep more than one male in less than a moderately planted 55 gallon tank.
Swordtails are not only a beginner's fish. They are terrific additions to any non-acidic aquarium environment. They are active, come in a vast array of color choices and can supply healthy, live food (by way of reproduction) for the adults and other carnivorous tank mates, usually in short order.