This is the first UnrealTournament first person shooter game starting in 1999 , wich is still working and available on Steam , but you have to install the latest patches to it , available here.
If you have fired up the patch the setup asks where you have installed the game , so you have to give in the path to the game within your steam directory.
Be sure your steam client is not running before patching/editing!
If everything went well fire up the game set video resolution etc ,
For multiplayer you find a lot of game servers to play on , happy playing!