Sometimes live brine will get a newbie fish to start eating. Try several different types of foods, too. It's a good idea to feed several types at all times, to try and help insure that the fish get all of the nutrients they need.
Before you buy any fish, ask the clerk in your LFS
to feed it before he nets it, and with you watching. Some wild caught fish just won't take to captivity and will die. If it eats in the store before you take it home, then there's a good chance it'll live.
Make sure that your tank has a few hiding places. Several rocks piled together makes nice dark crevices and caves for the fish to hide in. Many fish, especially Hippo tangs, need hiding places to feel secure, and especially when added to a new tank.