By default, when you start a computer with Windows 8, the default desktop will be the new Metro or Start. But you can start it with the classic desktop as the default one.
To see how you can manually switch between Start(Metro) and Classic desktops on Windows 8 check this tutorial.
Method 1: via Control Panel
1 Go to Control Panel
(to see other methods to access Control Panel in Windows 8 check this article).
2 Appearance and Personalization
5 Boot directly to classic desktop
Then click Apply and Ok.
Method 2: From Taskbar
1 Go to classic Desktop
(for other methods to switch between Start(Metro) and classic Desktop check this guide.
2 Right click on Taskbar
4 Change the default desktop
Next time, when you start computer, the default desktop will be the classic one.