You are going to find these 2 Support Article very important. First, view your Purchases History:
View your purchase history for the App Store and other Apple media services - Apple Support
Then review and cancel any subscriptions:
If you want to cancel a subscription from Apple - Apple Support
Any further action you take will depend on what has been purchased or if you saw a subscription to cancel. Of course you can use the provided link to try and request a refund, but if you have no idea how those occurred, then there is no reason why you will not see more in the future. You may need to change the Passcode on your device or Passcode on your account. If you have given this personal information to anyone else, then you have authorized them to make purchases, so you will need to correct that.
It may have also been a free trial that you started, but never cancelled. Free Trials are always converted to subscriptions if you do not cancel during the trial period.