You go to the start menu and right click on computer and select manage. In the window that comes up select disk management and then you will see a window that has all of the hard drives that are connected to the computer. You can right click the drive that you want to format and select format. You can also select delete the volume and then repartition the drive and you would still have to format so the easiest way is to just select format and make sure that you are selecting the correct hard drive.
If the hard drive was being sold then naturaly you would want to completely erase the old data but the OP is reusing the drive for a backup drive which once data is written to the drive the old data is gone.