Well....
I don't use Nero. I use Roxio Easy CD Creator. But I doubt Nero is any more complicated. There is a website with tutorials for Nero.
If you are using a CD-R disc you can only burn once, and after that it will be read-only. If you want a CD that is just like a high-capacity floppy disk then you need a CD-RW. You have to go to the store, e.g. office depot, and specifically buy a package of CD-RW discs. They should say "CD-RW" right on the package. These can be formatted and used to back up files in "drag and drop" fashion. You can use a CD-RW to store files as many times as you want.
However, if you have a DVD burner, don't bother with CD-RW discs. Just get a DVD-RW. They store like 10-times more data than a CD.