There are a number of ways. Assuming you have an original OS installation disk, you boot into the BIOS and set the boot order so CD is before the drive, then Save and Exit. Then during the boot process, watch for the prompt to boot to CD. You can then move into the Recovery Console, or start the install process for this copy Windows and follow the prompts partitioning and formatting the drive.

Alternatively, you could put the drive and install it as a secondary drive (NOT boot!) in another computer, then from My Computer, format it.

For more specifics, we will need more specifics from you about the hardware, and software you have.

If you did not get original disks with this computer, you will need to buy a Windows license, or go Linux.