I am being charged monthly for an app and I don't even know which one it is. How do I cancel it?

An effective way is to call your credit card company. Object to the bill. They have a "special" number for calling the firm that made the bill. In my experience, it's very effective when the credit card company makes a call to the firm.

There are instructions on this page for managing and stopping auto-renewing subscriptions (that should show the current subscriptions that you have and their prices, and let you stop them from renewing again) : Manage your auto-renewing subscriptions.