Morning
Glad you are keeping Skype

You need to go to Tools -> Options
In there there are 3 places you can change, the first is in the Privacy Tab, the second is in the Call Settings Tab and the third is in the IM & SMS Tab.
It should become clear when you look in these places but you can change the settings to, for example, 'Allow Calls from' ... 'people in my contact list only' instead of 'anyone'
Hope this helps

sgd