Go to Maintenance FAQ section and search for Diagnostic Codes. There is a thorough write-up. I found that I have to hold down the button for much longer than the book indicates in order to clear codes.

Remember that on OBD I, all you are really getting is an indication of the symptom that the system is able to perceive, not necessarily a diagnosis of the problem causing that symptom. Often, a code will be set because two electrical signals to the computer are out of whack with each other, such as one signal indicating that the engine is cold, and another signal that it is warm, at the same time. The OBD does not give you this much detail, it just flashes a code for one of the two offending signals (often turns out to be the one that is working properly).

Most common code will be oxygen sensor. If you oxygen sensor has over 150k miles on it, I'd just replace it before wasting a lot of time diagnosing.

My '92 had a bad connection between the fuse box and the oxygen sensor heater wire. I ended up stringing a new wire because I could not find the fault in the original circuit.

Sometimes they just set a code for no good reason, like when I go above 6000 ft and lug the engine down. Then you just reset it and keep going. If you have trouble resetting codes with the button, disconnect the battery for a few seconds. That will do it.