First of all, welcome aboard! Not sure I follow your question though. Restarting just means going up to the Apple icon in the upper left corner and selecting Restart.
But it sounds like you have some bigger fish to fry if you really want to restore it. Pop in your 10.5 installation disk, double-click the installer. When it reboots, you can select the option to erase and re-install. That means everything on the drive, everything, will be erased and a new operating system installed.