To play the game you have just to download extract it and launch "third_person.exe". You MUST launch Steam before launch the game or you will have an error when you want to host or join a party. If you want to join someone you have to have the same download region (Steam -> Settings -> Download -> Region of Download )
You can press U to change when you want your pseudo and you class.
You can press B to open the menu with graphics settings and change the keybinding or leave the game
If you press TAB the scoreboard will appear.