I need help with an unauthorized purchase

If you see ‘itunes.com/bill‘ or an unfamiliar charge on your bank, credit card, or debit statement - Does not mention purchases made under Family Sharing which might be charged to the organizer's card but will not appear under the organizer's purchase history or subscriptions. Ask family members about those or check your receipts. --> https://support.apple.com/HT201382

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

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

"The Report a Problem website shows most subscriptions, except those for Apple services and iCloud storage charges." [https://support.apple.com/HT201382]. You need to first cancel renewal on a subscription:

View, change, or cancel your subscriptions --> https://support.apple.com/HT202039

If you have unsatisfactory results with Report A Problem or you want to try to request a refund for a subscription, contact:

iTunes [Store] Support - Use this form for assistance with account and billing, code redemption, or purchase questions --> https://www.apple.com/emea/support/itunes/contact.html

or

Worldwide iTunes support --> https://support.apple.com/country-selector/itunes

You do not say where you see this purchase. If you can not see it using the methods discussed in the link(s) above there is the possibility you are being scammed.

Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - https://support.apple.com/HT204759

Contact iTunes Store support.