Boot from the CD/DVD installer.
Go through the process until you get to a point where you select the disk you want to install on, select the disk, click the Options button, and select Erase & Install.
If you want to erase it so that no data can be recovered, before starting the install procedure, go to the pull down menus until you see Disk Utility. Select it, and then erase the drive using the zero-all data option.