There is a lot to choose from. For most fish, the best all-around baits are nightcrawlers, and half a nightcrawler, or worms. Waxworms work well for sunfish. To catch carp, try a kernel of canned corn. Catfish are partial to a piece of turkey liver.
Minnows are great fish-getters.
If you don't know what type of minnow to use, just ask the person selling the bait. Take along a needle-nosed pliers to remove the hook from the mouth of any fish you catch. Worms, minnows, and nightcrawlers die easily, and when dead they will no longer attract fish. Keep worms and nightcrawlers in a cool, moist place, out of the sun. Put minnows in a bucket with a few small holes punched in the sides and keep it in shaded water nearby.