You will need to have a shop scan for "diagnostic trouble codes" stored in the computer (some auto parts stores will do this for you). The codes will give you a starting point for the diagnosis. Once the repair is completed, the codes need to be cleared.
Here is a good article on the Check engine light if you'd like to learn more about it, good luck.
If you post the trouble codes here, someone may be able to give you more information about what is wrong with your car