how can I find what subscription is costing me $9.99 a month?
Welcome to iMore! It's probably Apple Music. You can know for sure by opening iTunes Store on your iOS device, scrolling down to the bottom and selecting your Apple ID, tapping View Apple ID, tapping Manage Subscriptions, and checking there. You can also unsubscribe in that same spot.
If that doesn't answer your question, or if you feel this was done in error, visit
http://reportaproblem.apple.com to view all charges to your account and dispute any items you feel you shouldn't be responsible for.