If you just want to play a quick match versus some friends it might be easiest to use the menu Playerplayer/Create to start a server. Make sure you setup your router to do port forwarding to port 26000 on your computer and tell your friends to enter your external ip in the Multiplayer/Join dialog and hit the connect button.
If you want to run a 'serious' long running server you need a dedicated server. All the basic stuff you need for this is contained and explained in the directory Nexuiz/server you get when extracting Nexuiz. Read Nexuiz/server/readme.txt and do whats mentioned in there. Its just a few simple steps to get your server up and running. Really easy, i promise!
The fun begins when you want to run several well organized servers at once and also if you care for extra maps, map download or third party mods like nexball.
... more when i have time
