Instead of downloading Steam, open a terminal window (
Ctrl + Alt + T) and type:
sudo apt-get remove --purge steam
Press ENTER. Your password will be requested, type it and press ENTER again. This command will remove Steam if it was installed with errors.
sudo apt-get install steam
And press ENTER. Wait for the process to end, and steam will be installed on your computer, including all of the needed packages.
Note: I am posting this answer assuming that you downloaded Steam directly from your browser.
EDIT: If everything works, a Steam Launcher should appear in your Desktop. If this does not happen, open a terminal window and type
And then press ENTER.
Also, if you really want to install those packages, remove that :i386. For example, instead of
sudo apt-get install libgl1-mesa-dri:i386 use
sudo apt-get install libgl1-mesa-dri. Seems that i386 only indicates that you need a 32-bit package.