Once you start the game on the web, you can use the WASD keys to control one player, arrow keys to control another player, or Xbox controllers (any compatible controllers, really).
To start, when you're on the Player Ready screen you would press W to register for WASD keys, Up arrow to register for arrow keys, or start button to register controllers.
Once you have at least 2 people registered to play, you can press Enter key to begin the match.
In combat, your W and Up arrow key are essentially "gas" or "acceleration" to propel you towards where your tusk is pointing. Tag other Starwhal hearts while protecting your own!