apt-get originates with the Debian distribution of Linux. You'll need to use the package manager for your particular Linux distribution.
You could try typing "apt-get update" from the command line and see what response you get.
If you installed X Windows from X.org, you probably now have a lot of broken dependencies that apt-get won't like one bit.
It's best to install software through your Linux distribution, not directly from original sources. apt-get won't know about software you installed outside of the Debian package management system.