View your Purchase History here:
View your purchase history for the App Store and other Apple media services - Apple Support
You certainly don't need to have any subscriptions that will bill you monthly, you can check to see if you have any and cancel them by going here:
If you want to cancel a subscription from Apple - Apple Support
Remember if you sign up for a free trial you are agreeing to a subscription if you do not cancel the trial. This should be done at least 24 hours before the trial period ends to avoid any charges.
If you failed to cancel a free trial or made another purchase that was unable to be processed, that charge will not go away and will be deducted when it is possible to complete that transaction. If you owe money that has not been paid, it is likely that you will not be able to use the App Store until that matter has been taken care of.