Try to access the settings menu of the Skype application. Go to Video and select the webcam from that location. If the webcam is not available, try using Device Manager for its driver installation. If the webcam is not installed, you might need to install it using its driver before you can actually use it in Skype.
Embedded webcams have their drivers located on the support page of the device's manufacturer while external ones have their driver on the support page of their developer or on the Support CD.