If you're talking about a 3.5" floppy disk, then from Windows, click on
Start | Run | Open: command | OK. You should get a DOS window. Insert
the floppy into drive A:. At the DOS prompt, type: format a: /u
This command is difficult or impossible to undo, so be sure you really
want to format the disk. The /u switch means "unconditional"... it's
optional, but basically says to format the disk without first trying to
determine how it may have been formatted previously.
To close the DOS window when you're done formatting, at the DOS prompt
type: exit