I take it you want to create an image from an existing CDROM. To do that you need to mount that CDROM, say to /mnt/cdrom. Then you can create an iso image by issuing
mkisofs -J -o /home/user/yourcdrom.iso /mnt/cdrom
Now you should have an image called yourcdrom.iso in your home directory and you can mount this one like you would an actual cdrom in your drive by using the mount command.
I strongly suggest you
the manual pages for mount and mkisofs for additional options!