There is no Front Facing Camera (towards the user from the screen) on the Nokia Lumia 800, but there is a Rear Facing Camera (onthe back of the phone facing away from the user). The Lumia 900 has both Front and Rear Facing cameras.
You can start a video call with another Skype user by tapping the video call button (first button) in the application bar from the chat page. If you use the call button (second button) in the application bar, when the call has started, you can swap to a video call by tapping the video button (first button) at the bottom of the screen, and then tapping turn camera on.
When you are in a video call, you can switch between front and rear facing cameras (on phones that have both) by tapping the video button, and then tapping switch camera.
It depends on which device you have. Go to http://m.skype.com from your device and you should be able to download the correct version.