Account creation is a prerequisite to playing online. Your account will have all of its stats, and eventually items, saved to it. Account creation is done through the game client itself. When you first load the game, you will have two login options: Rez Account or Steam Account. A Rez Account is a standalone password-based account that is not linked to any third party platform. A Steam Account is a convenient option that will automatically verify and authenticate through Steam. When you first login using the Steam Account option, your username will randomly generate to something like SteamNoob_xfdsx. You can change this username either using the popup form, or by using the #rename command in the lobby. Once your Steam account is established, it will always be linked to your Steam profile. You can only have one Steam account linked per Steam user. If you decide to login using a Rez Account, you can leave the password field and one will be automatically generated for you. It is advisable to assign an email address to the account, because that will be the only way you can recover your account if you forget your password. It is also advisable to change your password once your email address is verified. Both account types are free and have the same features and capabilities. Once online you are free to play any mode, though it is recommended you start in the Practice Arena to get a feel for the gameplay and controls before jumping into competitive modes. There you will be able to bind your keys and adjustment controller or mouse aiming sensitivity to your liking. You can also practice against bots in the Practice Arena, and there are different difficulty levels to choose from.
The game is currently only available on PC, and the client is only available through Steam. You can find the game on the Steam store page here: https://store.steampowered.com/app/1934020/Rezarus/. Be sure to check the hardware requirements. Rezarus is not a demanding game, but being a solo-dev project it is also not highly optimized. There is a special Y2K mode for very low end PCs. Users with hardware that meets the minimum requirements have reported good performance on the Y2K and Low settings.