Use the Windows 10 Media Creation Tool to create USB installation media.
Then you can use it to install Windows 10 on your computer. You do not need to activate it immediately. When it asks for your activation code, just click "Skip". You can run this way, indefinitely, but you cannot change the desktop or the taskbar.
If you decide that you want to be able to customize Windows 10, then you can purchase a license and activate it.
-Wolf sends