This is the UnrealTournament III first person shooter game introduced in 2007.
This game is nowadays still working with multiplayer since the Gamespy shutdown now replaced by Openspy.
First of all you need the game installed available on Steam and manually patch it to the latest version available here.
You have to overwrite the existing ut3.exe within the Steam directory with this patched one.
Be sure if your steam client is not running before patching/editing!
Now you can register a new user for multiplayer, and play online again!
If you run your own gameserver you have to follow the above 1 step too.
If everything went well fire up the game set video resolution etc,
For multiplayer you find a list of game servers to play on , happy playing!