To install steam, you have to use wine (which is a little hard to grasp at first) Cedega, PlayOnLinux, or Crossover Linux.

https://help.ubuntu.com/community/Games/Emulators

There are tutorials out there on how to install CSS and Steam on Ubuntu. Plenty of guides out there. Its very buggy though. It won't run like you remember it but you still get to play it.

You'll get the hang of it. You and your terminal are about to be the best of friends.