My Apple ID was used to make unauthorized purchase

If you are seeing the purchase on your iTunes account's purchase history (e.g. it's not just a phishing email that you've received) contact iTunes Support (I assume you've already changed the account's password) : http://reportaproblem.apple.com or http://www.apple.com/support/itunes/ww

Do you receive email notifications from Apple to say what you are being charged for?

If so, click on the Apple email address. If the resulting drop down shows a bogus or unknown address, it isn't from Apple.

Also, if you do online banking, check to see if any debit has been charged that corresponds with what the email says, but only after 24 hours after the email was received. Your bank may not have caught up with the day's transactions.

Failing all that, take the matter up with Apple AND your bank to obtain satisfaction on the matter.