You do not say where you see this purchase. If it is an e-mail 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: email@example.com
If you still 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.
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 recognize this purchase and are asking for a refund then realize Apple's terms of purchase in the USA are, "All Transactions are final" (https://www.apple.com/legal/internet-services/itunes/us/terms.html), but some countries have different laws so check terms for your country at: http://www.apple.com/legal/internet-services/itunes/. If sales are final then it is Apple's decision if they want to grant a refund. You can always ask, and people do get refunds on a case by case basis, but Apple can also say no.
Get help with an item purchased from the App Store, iTunes Store, or iBooks Store - 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"
Other ways to contact iTunes Support: