Yes
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.
Yes.
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 install a steam shower for yourself, however, you need to be prepared to do a lot of work. You need to steam-proof the tub you are converting, prevent the steam from escaping, collect and distribute the condensation away from the user, and allow it to drain. There are lots of instructions that can be easily found online for how to do the installation.
No, once you purchased them you will have them after re-install process.Whatever you buy on your steam account stays with your steam account. You just need to log-in with your steam account and your dlc skins should be installed automatically to your computer.
Go to the steam store here is the link store.steampowered.com/app/218620/ You need to install steam and make a account for it
Team Fortress 2 is Free-To-Play on PC, so you do not need any CD. You need the software Steam to install the game, though.
You will require internet to install the game; as it is on steam. Apart from that it does not need to be connected to the internet.
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.