Some phones come with a QR scanner pre-installed but others require you to download an app. There are loads of apps available but you need to take care, as some are supported by ads. If you need a QR scanner for an Android phone, I can recommend the Trend Micro one. (No ads).
You simply turn the app on and point your phone's camera at it. The app will check the link for safety and then take you to the relevant web page.