First make sure your /etc/apt/sources.list contains the wheezy-updates repository (this makes sure you always get the newest updates for the ClamAV virus scanner – this project publishes releases very often, and sometimes old versions stop working), and that the contrib and non-free repositories are enabled (some packages such as libapache2-mod-fastcgi are not in the main repository).
To edit them run the command
nano /etc/apt/sources.list
If you also want the contrib and non-free components, add contrib non-free after main. like below
deb http://ftp.de.debian.org/debian/ wheezy main contrib non-free deb-src http://ftp.de.debian.org/debian/ wheezy main contrib non-free deb http://security.debian.org/ wheezy/updates main contrib non-free deb-src http://security.debian.org/ wheezy/updates main contrib non-free # wheezy-updates, previously known as 'volatile' deb http://ftp.de.debian.org/debian/ wheezy-updates main contrib non-free deb-src http://ftp.de.debian.org/debian/ wheezy-updates main contrib non-free
Update Debian Source List Installation