Refund for what ? If it's an app and you've tried deleting it and re-downloading it via the Purchased link (re-downloading) and, if that doesn't fix it, tried contacting the app's developer then you can use the 'report a problem' link from your purchase history : log into your account on your computer's iTunes via Store > View My Account (Store > View My Apple ID on iTunes 11) and you should then see a Purchase History section with a 'see all' link to the right of it ; click on that and you should see a list of your purchases ; find that app and use the 'Report a Problem' link and fill in details about the problem (iTunes support should reply within, I think, about 24 hours).
Similarly you can use the 'report a problem' for films, tv shows, songs that don't play.
If it's something that you purchased by mistake then all purchases are considered final, but you can try contacting iTunes support and see if they will refund or credit you : http://www.apple.com/support/itunes/contact/ - click on Contact iTunes Store Support on the right-hand side of the page, then Purchases, Billing & Redemption
If it's a different reason ... ?