I'm gonna assume that you have a working computer of your own. if this is the case then there is one of 2 things:
1. You bought a pre built system with OS installed already and you got backup disks etc
2. You built your own rig and installed the OS your self
In either case you have OS disks (recovery disks are just fine).
Pop them in your drive, boot up from cd/dvd and there is always the option of format on them, just use that, formatting is the same no matter how its done