You do not say where you see this purchase. If it is an e-mail are you sure it is real?
Identifying legitimate emails from the iTunes Store - http://support.apple.com/kb/HT2075
Phishing & Other Suspicious Emails - https://www.apple.com/legal/more-resources/phishing/
Forward attempts to: firstname.lastname@example.org
If you think the charge is real:
See your purchase history in the iTunes Store on a Mac or PC - http://support.apple.com/HT204088 - Does not include purchases made by others under Family Sharing; those have to be checked from their accounts.
iTunes Store charges on credit and debit card statements - https://support.apple.com/HT201382 - "See an unfamiliar iTunes Store or App Store charge? Before you contact Apple, check your iTunes Store purchase history for more details."
If neither you nor a family member (if applicable) has a record of this purchase and you only saw it on a credit card statement contact your bank or card company about card fraud. Apple does not handle that.
Report a Problem - http://reportaproblem.apple.com - "Sign In for help with iTunes purchases made in the last 90 days"
Other ways to contact iTunes Support: