You are sure that it’s a genuine email from Apple and not a phishing attempt trying to get your account and payment details ? Does the email contain your full billing address from your iTunes account ?
Phishing and similar emails : Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support
Genuine iTunes emails : Identifying legitimate emails from the iTunes Store - Apple Support
Viewing an account's purchase history : See your purchase history in the iTunes Store - Apple Support
If it's a genuine email and if it shows on your account's purchase history, and if nobody else has access to your account and did it, contact iTunes Support : https://support.apple.com/contact . And change the account's password http://appleid.apple.com - after changing it : What to do after you change your Apple ID email address or password - Apple Support.
If it's a phishing email you could forward it to Apple : reportphishing@apple.com , and then delete it.