Change your password.
Remove your credit card from the billing information in your account.
If the charges were by an unknown 3rd party, call your credit card company to dispute the charges. In some cases they may ask you to follow up with a police report.
If the charges were by a family member who was not authorized, contact iTunes Customer Service at http://www.apple.com/support/itunes/contact/ . Explain the extenuating circumstances, and request a refund. They are not obligated to provide a refund, but will sometimes do so on a one-time basis if there is a reasonable explanation.
For future reference: The iTunes Store allows an app to list itself as "free" if the initial download is free. Many "free" apps contain offers/inducements for later in-app purchases. In-app purchases primarliy consist of worthless, overpriced items such as "jewels" or "brains" used in some game. If any underage person is using your device, make sure to turn off access to the store, and to implement all possible iOS restrictions: