You need steam to install Portal 2 Why not? It's free
Portal or Portal 2 mods on Steam is installed by first Downloading it and extracting it. Then copy the map file into your Portal 2 maps folder which is Run the game.
Here's one place you can get it from (you'll need to install steam first though): http://www.steampowered.com/nvidia/
Steam will give you a unique code.
It is free on Steam until the 20th of September. Just go to the Steam store and search for Portal. It says it costs money but you can install it without paying. Enjoy it, it's a very good game.
Yes it is. You need the Steam application to download it.
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.
If you want to play the game on a PC or a Mac, yes.
Yes.
The program Steam allows you to buy it, and many other games. go to steampowered.com to download Steam, start Steam, search for Portal, hit Purchase Game, and download it
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)