Someone used my account to purchase an app

If the only indication of the charges is from an email or text, be aware that it may be a phishing attempt to get you to log in to a site that appears to be Apple and divulge your password or credit card information. See: Avoid phishing emails, fake 'virus' alerts, phony support calls, and other scams - Apple Support

and Identify legitimate emails from the App Store or iTunes Store - Apple Support

If it is a real charge, see the following instructions. They include information on contacting Apple Support if the other steps don't help: If you see apple.com/bill, itunes.com/bill, or an unfamiliar charge on your statement - Apple Support

Then your Apple ID was compromised. See this Apple support article—>If you think your Apple ID has been compromised - Apple Support