If you have an autozone in your area they will read them for free.
if not, you can buy a code reader for as little as 40-100 bucsk. A shop will charge 50 bucks to read the code, so it is a great investment.
I have this one:
http://www.harborfreight.com/cpi/cta...emnumber=46030
I recently used it on a friends car that had a CEL and we found out what the problem was.