unauthorized itunes purchase

If you didn't do the purchase, and nobody else who has access to your account or devices made it, then try the 'report a problem' page to contact iTunes Support : http://reportaproblem.apple.com

Or you can try contacting iTunes Support via this page : 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

High,

First make sure there really was a purchase. A common form of phishing is an email claiming a large purchase was made on your account, as a way to scare you into clicking on a fake link.