Do just what apt-get says, and run
That forces the install without checking dependencies. Once that is done, run apt-get update and apt-get upgrade, and everything should be cool. If something gets screwed up, run apt-get dist-upgrade. If you downloaded the gcc .deb file from the debian website and installed it, that was not a good idea. Gcc is the basis for a ton of programs, and it has a ton of specific dependencies. Always let the apt-get system handle major programs like gcc, so that you don't get into trouble.
You add repositories to /etc/apt/sources.list. You can open that file and see the format. It looks something like
deb http://ftp.debian.org/debian/ unstable main non-free contrib
deb-src http://ftp.debian.org/debian/ unstable main non-free contrib
deb http://download.videolan.org/pub/videolan/debian sid main
deb-src http://download.videolan.org/pub/videolan/debian sid main
Where I have unstable/sid you may have testing/etch or stable/sarge.