Unwanted purchase

Check the message or email you received:

In a message Apple will always address you by your real name, never by your email address or Dear Customer. Also the sent-from address should be @apple.com or @itunes.com Additionally Apple does not send attachments.

Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

Forward it as an attachment to reportphishing@apple.com

Are you sure it was charged to your account, it may be phishing?

Check your purchase history:

See your purchase history in the App Store or iTunes Store - Apple Support

You have to contact Apple support immediately thru Phone or Web Chat, and they will check the purchase and how it was done, you may have a refund of this amount as well.