Take it over to orielly or one of the other chain auto parts stores... They'll hook up their OBD reader to it for free, tell you the code or codes that are set, and give you the option to clear the code. If it's a real problem, the light will come back on in the next trip or two.

The code or codes can point you to the defective part, but don't just change out whatever part the code says.. for example, if it says "P0171 O2 Sensor System Too Lean (Bank 1)", don't replace the o2 sensor, since it's probably not the problem, just the part that is detecting the issue.