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 now 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"
if a reporting a problem request is unsuccessful you can also try:
other ways to contact itunes support: