it depends what opperating system you have.
if you have windows 98 then you should put in the boot disk or boot CD when you get to DOS you should type format c:
If you have windows NT/2000/XP then boot from either the disks or from the CD and use the tools in the setup program to format the disk before you install the OS.
If you are talking about formatting over an OS in order to resell a business disk then I recomend that you use dban
which will securly and irrecoverably erase the disk for you (It's a free tool).