After the pond has cycled, small goldfish, mosquito fish, or shiners for a smaller pond. A larger pond can house bream, koi, catfish. Just gotta make sure you have enough oxygen to support the species you choose.
Small ponds overheat easily so it can really put a damper on the species you can keep.
A newly created pond needs to "cure" or "cycle" as Isadora mentioned. That means it needs to operate for awhile before adding any fish. This can takes a few weeks or even a couple of months. Adding fish before hand will surely kill them. Once the water tests for appropriate Ph, oxygen and temperature, adding small fish as testers for the aqua environment and letting them live for awhile before adding other more expensive fish is a wise idea.