The Steam client can be downloaded from their official website, linked below.
You need steam to install Portal 2 Why not? It's free
Yes
If you have purchased the game on Steam then you need to install Steam on that PC and there should be an install button on the game in your Steam Library. If you have the disk version you just have to put in it and install it using the CD key that came with it.
In the games tab, click "Activate a product on Steam..." to install the game if you have the CD. If you bought the game from Steam, you should see it on your games list. Right click it and install it.
If you bought the game from steam, no. and if your trying to put a game on your computer and it keeps adding to steam, than un-install steam while your trying to install the game.
Steam
if you have steam you can install it from there but if not you will manually have to place the files in the correct place
Civilization 5 is powered through the Steam system so you will need to install Steam and create an account to install and run the game. (See links below)
You can buy it from Steam or install it with the CD.
Yes.
You can get it off of Steam.
You get it from the store or steam if they have it.