A wide selection of packaged, frozen, and live food is available for feeding your fish. Some specimens, such as my teacup stingray, must have live food. Most freshwater tropical fish do well on flakes and pellets, though I have always treated my fish to live food at least 2-3 times a month. For saltwater fish, brine shrimp and other frozen fish foods are best.
You can buy all of these fish foods at your local fish store. Here's a neat idea for aquarium hobbyists: the "pet sea monkey kits" sold as novelties are actually nothing more than brine shrimp eggs and instructions for hatching them. I have kept these sea monkey kits from time to time so I could grow my own brine shrimp for my fish.