Check that the non-free repository is enabled by inspecting /etc/apt/sources.list
.
sudo nano /etc/apt/sources.list
If non-free is not included then modify the file so that it does.
deb http://http.us.debian.org/debian main non-free
Update your system.
sudo apt-get update
You can now install the package :
sudo apt-get install libapache2-mod-fastcgi