updating an old mac OS X 10.4
The latest supported OS on your computer looks to be 10.4.11
(looks like you have a Quicksilver (PowerMac3,5))

And if I'm not mistaken, Microsoft dropped Skype support for everything before Lion (10.7)

You may be able to get an adapter to plug your monitor in, depending on what that large plug is. Though that is really a separate question.