How do I get them:

For a 1997 TJ:

Turn the ignition key ON THREE TIMES in succession:
ON : OFF : ON : OFF : ON
(within FIVE seconds)

Count the number of times the check engine lamp on the dash flashes. The number of times it flashes is the error code. There will be a short pause between the flashes representing the different digits of the code. Longer pauses separate individual trouble codes. These codes are 2 digits only. Code 55 (*****_*****) will be the last code to be displayed.
Here is what they mean

For 1998 - Present TJ's

In 1998, Chrysler changed the way codes are read in their vehicles.

On TJ's with the newer diagnostic system, place the key into the ignition, push and hold down the odometer reset button, turn the key to RUN (do NOT start), then release the odometer reset button. When you do this you get a test of the dash functions and any error codes will display in the odometer area. These codes are 2, 3 or 4 digits.
Here is what they mean