Most OS Install discs have the ability to format your computer easily, if you just want to format it with out a OS disc, download and burn UBCD. UBCD is a formating/fixing tool disc that will allow you to format your HDD or try to fix the os installed.
For Windows / Linux 1) Turn on the computer 2) Insert the OS disc into your dvd/cd drive 3) When computer is done booting from OS disc, select the install option(this will format the hard drive removing all previous data. 4) follow on screen instructions