How big of a tank do you have? 55g or bigger I hope.
Serrasalmus nattereri, or the Red-Bellied Piranha requires a generally live-food diet. As fry they require baby brine shrimp, and as they get larger (.5" to 2.5" in size) they should be fed a mixture of flake food, live foods (brine shrimp and various worms),frozen foods (brine shrimp, beefheart, bloodworms, daphnia, other carnivorous foods). As they become adults, they should be fed beef, other meats and live foods: goldfish, earthworms, sometimes even pinkies or rats, although hair will affect their digestive system. Steak and some live foods will foul water quality faster, so be ready for water changes.
Aquascaping should include a thick layer of gravel with plants such as amazon swords, thick floating plants such as hornwort to block some light (piranhas like lower lighting conditions than most fish). Driftwood is also nice, if you keep plecostomus in the tank (common plecostomus, which are one of the few fish able to be kept with piranhas, they are large and cheap so most people are willing to risk losing them), they like to nibble on the driftwood. Full grown fish can reach 8-10 inches, so a large tank (20 gallons for one, 55 gallons for about six, 75 and up can hold more) will be required. They can live long if well taken care of, 5 or more years in captivity at most.
http://www.fishinthe.net/html/fishguide/fishguide_fish.php?FC=59&nl=&nt=1