In iPhones (iOS 11 or better) and certain Android phones from Xiaomi and Motorola you just take a photo and it will detect a possible QR code and open a browser if there’s a link. The way it should work.
In Android phones you can otherwise use Goggles (often pre-installed) and Google Now! to read QR codes.
For other phones we recommend using CliqTags QR Code Reader, as it’s very simple and to the point.
You can also use any of the free QR/bar code readers available for smartphones.