Corals can grow on just about any typical rock, but usually the best choice is a piece of live rock. Some this also depends upon the coral you want to grow. You can start them on a small piece and later epoxy the small piece to something larger once you get some growth on the coral.
Keep in mind there are a few species that don't live on rocks, and for other corals, the method to get the coral to attach will depend upon the coral. Hard corals are usually best attached using superglue or epoxy. Soft corals are usually best attached using the toothpick and rubber band method.