That's right click taskbar,
Properties and change
Taskbar buttons to Always combine, Hide labels. Which will affect every taskbar button.
OR
- In Skype, go to Tools -> Options.
- Then, click on Advanced tab to go to Advanced settings.
- Uncheck and untick the Keep Skype in the taskbar while I’m signed in option and click Save.
When you use the "Red X" to close the Skype window it will now minimise to the System Tray.