Pokémon Masters is a collaboration between Pokémon Company and DeNA, the mobile game company that also developed Super Mario Run and Fire Emblem Heroes. The companies released a bunch of gameplay videos when pre-registrations opened that show how sync pairs work, as well as the battle mechanics and the moves available in the game. You may want to check them out to get an idea of how to play, though you can now just jump right in and figure things out as you go on your journey to train and compete in the League.