To create an ISO image from your CD/DVD, place the media in your drive but do not mount it. If it automounts, unmount it.
# dd if=/dev/cdrom of=/path/til/ISO-fil
To make an ISO image from files on your hard drive, create a directory which holds the files you want. Then use the mkisofs command
# mkisofs -o outputfile.iso /my/folder/
The resultfile is called outputfile.iso witch contains all the files in the /my/folder/ folder.