Are you sure the payment was actually taken, as opposed to getting an email telling you the purchasse had been made? - such emails are scams and shouldn't be responded to (in particular never follow any links in such an email).
If you have checked with your card issuer and the list of purchases in iTunes and there actually was a fraudulent purchase, change your password (log out of iCloud and iTunes before doing so then log back in once it's changed). Then contact Support:
iTunes Store - Contact Us - Official Apple Support