Boot from your grey installer DVD disc 1 (hold down the C key on startup or hold down Alt/option on start and choose the installer disc).
OK the language page (if present). From the installer screen, go to the menu bar and choose Disk Utility; depending on the OS version it may be in the Utilities menu or Tools menu.
In DU, select your internal drive in the sidebar (the top item with the makers name and serial no.). Run Repair Disk. If that comes up as disk OK, click the partition tab. Select the partiton from the drop-down above the graphic; 1 partiton is all you need. Go to the options button and ensure that the partition scheme is GUID (for Intel Macs) or APM (for G5 and earlier) and the file system to Mac OS Extended (Journalled). Name the partiton (usually Macintosh HD), click Apply.
When that's finished, select the new volume in the sidebar (indented to the right below the drive) and go to the Erase tab, select Security options and select zero data (one pass is more than sufficient). Click erase. That will take quite some time; probably measured in hours and dependant on the size of the drive.
When that's completed, close DU and continue with the installation.