Please use the search function. This has been asked multiple times. Either join a game and type "menu setup_funname" in the console. If you don't how to get there hit the button under esc that has ` and ~ on it. Instead of typing that you can just go to the main menu, hit play, then multiplayer setup then advanced which is located next to the name box. If you get the "u and s" problem
read this and the post by Zorch will help you.