Someone has used my account to make a purchase what can I do ?

It shows on your account's purchase history : See your purchase history in the iTunes Store - Apple Support ?

i.e. it's not just a phishing email that you've received : Identifying fraudulent "phishing" email - Apple Support ?

If it does show on your account's purchase history and you haven't already done so then change your account's password e.g. by logging into it via the Store > View Account menu option on your computer's iTunes, or by tapping on it in Settings > iTunes & App Store on an iOS device and logging into it.

And then try the 'report a problem' page to contact iTunes Support : http://reportaproblem.apple.com