It depends if you are wanting to format it in Windows or through the DOS shell.
If its in Windows then just highlight the drive in My Computer and right click your mouse to bring up the menu and highlight format !
Provided it's not your c drive. If so you'll need to boot from a dos floppy or windows startup disk and use the format command. Then (re)install the OS.
To format a c: drive you will need to insert a System disk,when it has loaded to the a:prompt type ' fdisk ' then choose option ' delete primary dos partition ' when this is done choose option ' create primary dos partition '.
When you've done this reboot your pc,when you get back to the a: prompt type ' format C: ' and enter.
You will then need to load on your operating system.