I downloaded an app that I don't want and paid for it, how do I see how much it cost and receive credit?

All sales are final. You don't "get credit". Not wanting it is not an excuse. If you didn't want it, you shouldn't have downloaded it. If there is something wrong with the app or it doesn't perform as advertised, contact the developer. If you can't work it out with them, then contact iTunes store support. They may be able to do something for you.