How to update and install packages on Debian
There is much more on all of this, I recommend reading as much as you can.
There is not much details in you post,..like what version of Debian ?
If this is a fresh install, you need to edit your /etc/apt/sources.list
file,... the lines for CD should be commented out, or removed.
Unless you are trying to use a cd to update ?
It would help to see what your actual /etc/apt/sources.list file has in it.
There is a example of what it should have in it here:https://wiki.debian.org/SourcesList