i keep being billed for an app i didnt purchase. What do I do?

If you mean that you are being billed periodically (e.g., monthly) it is likely a subscription. See the following: View, change, or cancel your subscriptions - Apple Support

You can also use the following instructions to report a problem: Get help with an item purchased from the App Store, iTunes Store, or iBooks Store - Apple Support

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. 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

That is a phishing scam. It is not from Apple. Don't respond to it. See the last paragraph of my previous reply.