If these charges are being made to a credit card, the most immediate thing you need to do is to call your credit card company and report the unauthorized charges. Your card company can contact Apple if they need additional information.
Next, immediately change the password to your iTunes Store account, making sure it's not something easily guessed or looked up. Then go here:
and follow the instructions to report the issue to the iTunes Store.
If the charges are not reflected in the Purchase History, then it's quite likely that they are fraudulent charges made to your credit card directly rather than an unauthorized purchase through your iTunes Store account. In that case it will be up to your bank to handle.