To Install Windows 8 without installing Windows 7 / Vista first, you’ll have to download the Upgrade installer to create an ISO or a bootable USB Device.
You can find the Upgrade installer here: http://windows.microsoft.com/de-DE/windows/buy
After this you’ll have to start the installer as an Administrator.
The tool will download Windows 8 Pro and asks for an Installation type.
There you can choose between USB device installation or to create an ISO file.
Now you can install Windows 8 Pro.
After finishing the Installation, you’ll have to activate the Windows 8 Pro.
If you haven’t upgraded this Version from an previous, you’ll have to do a little trick.
Open Regedit (press WIN+R and enter regedit and press enter)
Search for the key HKEY_LOCAL_MACHINE/Software/Microsoft/Windows/CurrentVersion/Setup/OOBE
change the value of MediaBootInstall from 1 to 0
Open the Command line as an Administrator (press Windows Key enter CMD and rightclick on it and choose open as administrator)
Enter slmgr /rearm and press enter
Reboot your Windows.
After your device has rebooted, it should be activated.