Any free subscriptions you start in the app store will become paid when the free trial runs out unless you stop the renewal at least 24h before the renewal.
To stop a subscription follow the steps in this guide:
How to cancel a subscription from Apple - Apple Support
When the subscription shows Expires on [Date]
That means the subscription is canceled.
If you are still unable to cancel the subscription:
Contact Apple for support and service - Apple Support
You can request a refund by following the steps in this guide:
Request a refund for apps or content that you bought from Apple - Apple Support
You are not guaranteed a refund tough.
This is an automated process, if you wish to explain your situation in-depth to a person you can:
Contact Apple for support and service - Apple Support
Also, see this:
How long refunds from the App Store or other Apple services take - Apple Support
Good luck!