If you are in a region where the beta for Pokémon TCG Live is available, you can install the game on mobile devices via the Apple App Store and Google Play. (Note, these links take you to web pages not created or maintained by The Pokémon Company International.) For Windows and macOS devices, you can download the app from Pokemon.com/TCGL.
For information about system requirements, please check the Pokémon TCG Live website, here.