If you have noticed unauthorized charges from Skype on your credit card bill, Paypal account or bank statement, then you may be the victim of fraud. There's a fairly quick checklist that you can go through to be sure.
First, make sure that it's actually fraud. Skype does offer automatic refilling for your credits. While you may not have put through an order to buy more, it's possible that you checked the box previously. I suggest quickly checking your account before you panic (plus, they'll probably ask anyway.) You can check this by pulling up your settings, and clicking the Payment tab. If automatic recharging is enabled, there will be a note on the side verifying the level that you set.
If you don't have a Skype account, have never bought Skype credits or have verified that it isn't a legitimate charge, then you will need to get the payments stopped. Reporting fraud to Skype isn't actually that hard, especially if you have an account set up with them already. Just go to https://support.skype.com/en-us/search_first/. You'll need to search first before they let you go through to the contact page, so check the FAQ and then click on Get More Help at the bottom of the page. Then just log in with your account name and choose how you'd like to contact them.
If you don't have a Skype account, then you should contact them at firstname.lastname@example.org, and report the fraudulent charges to your account.
For either method, they'll need the last 4 and first 6 digits of the card, your name, your country and the order numbers associated with the charges. Note that this will only be a stop on the account and note it as fraud to Skype. You still need to contact your credit card company using the number on the back of the card for the fraud line, and report the charges. You should probably also have a new card issued, since your number has been compromised.
If you take these steps, then Skype will note the fraud and the charges should be reversed according to the company's policies.