A diagnosis will have to be performed to find out the cause. Codes will be found stored in the Powertrain Control Module - the engine's computer. These codes offer clues as to what is wrong with the car. Tests are performed based off the found codes to determine the cause of failure.
One simple thing you can do - make sure your fuel cap is secure. If the gas cap is loose, the check engine light can come on as a direct result. Secure the cap, and allow the engine light to reset on its own eventually. If the cap is tight -- follow through with getting the diagnosis done.
Check this link to find shops near your that can help.
...with 120,000 miles. I had the timing belt, Serpentine belt and Valve cover gasket changed. the engine light came On for "Camshaft sensor". i drove the car home after the mechanic diagnose my car