You will be asked what you want to download:
First you choose what version (i.e. Pro, Home, etc.)
Then you select to download to either a USB drive or an .ISO file.
Lastly you choose to download for 32 Bit, 64 Bit or both.
Also, you can use this tool to perform an upgrade from 7 or 8.1 if you want. This way you won't have to wait for Windows Update to offer the upgrade to you.
P.S. You won't need a key to do the upgrade.