Thank you for wanting to try the game! To play it on PC you'll need to download the PC file and have some kind of file unzipper, like Winzip or 7zip. Then you just left or right click the downloaded file, depending on how things are set up for your unzipper, and have it unzipped. That'll get you a folder of stuff. Inside the folder will be an icon which you can click to start the demo.
For more direct help, try searching "how to unzip a file on [whichever type of PC you're using]" and then you'll be able to find more specific guides/videos. I hope that helps!