The easiest way is to stop by your Toyota service center and ask for the manager. See if he'll take 45 seconds and hook up their diagnostic monitor to the terminal under your steering wheel and tell you the codes and what they mean. Otherwise, go to mkiv.com and follow the instructions. Under the hood near the firewall on the passenger side is a little black box with a flip-up lid. You basically bridge two of the terminals together and count the "check engine" light in the dash as it blinks. For an error code of 21 it will blink twice, then pause, then blink once.
Good Luck,
John
www.utahracing.com/supras/john.html
P.S. If you have removed your factory downpipe, it's almost a 100% sure bet that the code will read catalytic converter failure.