Open iTunes on a computer (if it's someone else's computer, remember to log out whenever you're done). Go to the iTunes Store, click Account, and then click "See All" beside the line titled Most Recent Purchase.
If you see unauthorized purchases, as other posters have suggested, it could be an unauthorized person accessing your account or perhaps children making in-app purchases in games. You can also check your Apple ID email for receipts, since Apple sends receipts for every non-free purchase they complete, as far as I know
be it iTunes content, App Store apps and in-app purchases, Apple Store purchases, or iCloud storage plans.
I'd recommend you change your password to something stronger immediately no matter what, and contact Apple to see about getting those charges reversed. They're usually pretty liberal with refunds as long as you don't make a habit out of it.