Unauthorized charges

You do not say where you see this purchase. If it is an e-mail or text message are you sure it is real and not an attempt to steal your private information ('phishing') (very common)?

Phishing & Other Suspicious Emails - https://www.apple.com/legal/more-resources/phishing/

Identifying legitimate emails from the iTunes Store - http://support.apple.com/kb/HT2075 (the previous link is more specific)

Forward attempts to: reportphishing@apple.com

If you still think the charge is real:

See your purchase history in the App Store or iTunes Store [iOS, computer, or web] - Does not include purchases made by others under Family Sharing; those have to be checked from their accounts.

http://support.apple.com/HT204088

If you see ‘itunes.com/bill‘ or an unfamiliar charge on your bank, credit card, or debit statement - https://support.apple.com/HT201382 - "Before you contact Apple to request a refund for a charge you don't recognize"...

If neither you nor a family member (if applicable) has a record of this purchase and you only saw it on a statement for a card you alone control, contact your bank or card company about card fraud. Apple does not handle that.

If you do now recognize this purchase and are asking for a refund then realize Apple's terms of purchase for most countries are effectively, "All Transactions are final" (check terms at: http://www.apple.com/legal/internet-services/itunes/). Apple does refund on a case by case basis, but they can also refuse.

Request a refund for an App Store or iTunes Store purchase - https://support.apple.com/HT204084

Report a Problem - http://reportaproblem.apple.com - "Sign In for help with iTunes purchases made in the last 90 days"

If a reporting a problem request is unsuccessful you can also try:

Other ways to contact iTunes Support:

https://www.apple.com/emea/support/itunes/contact.html

or

https://support.apple.com/country-selector/itunes