Julie here from Shopify Support.
If you're receiving monthly charges from Shopify, there's a good chance that you had possibly started a free trial at one point and had entered your billing information at that time. The easiest way to determine whether this is the case is by using the store recovery tool. There, you'll be able to enter the email address you would have used when you signed up, and you'll be provided with a list of any stores that match that email address.
If you're unable to find any stores associated with your email address using the store recovery tool, then you'll just need to contact Shopify Support directly. They'll be able to help you track down your store and ensure that it's closed, preventing any further charges.
I hope this guides you in the right direction. Best of luck!