Steam is the world's largest digital distribution platform for games and software.
If you haven't done it already, you will need to create a new Steam account.
It's free and doesn't require any credit card details.
Go to the Steam website and click on the green 'Install Steam Now' button.
Install and launch the Steam client and create a new Steam account if you don't have one.
After installing Steam, you can download the demo by clicking on this button: