If you haven't played Clash Royale before, you'll need to download it from either the Google Play or App Store first (depending on what device you're using). From there, select your preferred language and country and whether or not you want to create an account with Supercell (this isn't necessary, but if you create an account, then any purchases you make in-game will be saved so that they can be accessed later). Once everything has been set up, tap "Play" to launch the game for the first time.