1999 Ford Contour Check Engine Light

There is no problem with the way the car runs, but the check engine light comes on most of the time. What can I check on my own to find out the cause of this problem?

The oil is changed every 3,000 miles.
The alternator was replaced about 4 years ago.
The rear struts were replaced 6 months ago.
The air conditioning only blows air out of the passenger side.

I would appreciate any suggestions you can give me to help resolve this.

Thank you. Robert Crawford