This really isn't a Word question, but...

You format a diskette by putting it in the drive, right-clicking its icon in
Windows Explorer, and selecting Format.

If it's a floppy disk that you've been using for a long time, then there
probably is something wrong with it. They just aren't very reliable.
Fortunately, they are cheap -- throw it away and use a new one. If you get a
lot of formatting failures, maybe the disk drive is failing. They're cheap,
too, and you can replace it with only a screwdriver and a little
instruction.