To clear the contents of
parameter RAM upon starting up your (pre-
OS X)
Macintosh, hold down
Command-Option-P-R before restarting/powering on. You will hear and see the computer reboot again in a few seconds, at which point you may let go of the keys.
"Zapping" the PRAM is not usually necessary, but may be a useful thing to do if you are having serious problems with extension/control panel settings, and wish to reset their information.