How can I cancel a charge that I made by mistake?

You are not speaking to Apple, this is a user to user community. 

We are unable to help with refunds, account issues or purchases  

Cancel/see your subscriptions here - See or cancel your subscriptions - Apple Support 

Try applying for a refund here, but there is no guarantee it will be refunded - Request a refund for apps or content that you bought from Apple – Apple Support

Or Contact Apple Support - Contact - Official Apple Support