Recommend me a singleplayer shooter

Farcry 1 was good as well.

Also, C&C Renegade is actually usually acceptable to download from somewhere. It has a Tutorial, a Practice with NPCs, and a lan and internet game mode.

Internet game mode still has dedicated servers up on Westwood Online. HOWEVER, the serial for the game is the specific part. You can't gen a key that will connect to Westwood Online. Thus...

You must either buy from XWIS Strike Team a working key, OR you must connect to Gamespy Arcade, which isn't as good but still has dedicated servers and mods and players.

Honestly, if you can get Gamespy Arcade, you could play for free, and their community will probably exist for longer since if they are still there 10 years later then they probably aren't going anywhere.