To start a multiplayer game (to fight against human players), choose "Multiplayer" from main menù. Privacy notice: please keep in mind that your activity while connected on a server (e.g. what you "say") may be saved by server logs (and sometimes used for statistics), and that other players may save console "dumps" that may include chat lines from yours, or may record demos including you amongst the other players (even espressly spectating/following you). After reading this page, you can go deeper by reading Manual/Multiplayer/Advanced, especially in case you wish to be a server admin.