When the Service Engine Soon light comes on, it's the on-board computers way of telling you it sees a problem in one of the monitored systems. If you are not experiencing drivability issues (surging, stalling, rough idle), then more than likely it is a problem in an emissions system. Unfortunately, there are litterally 100s of sensors monitored and to take a "guess" would just be inefficient and a waste of time and money. A good place to start is at your local parts store (ie. NAPA, AutoZone, Advanced). Most of these stores will read the "p-codes" from the on-board computer for Free in hopes of selling you the part/sensor you need to make the repair. Once you have this p-code, it will tell you which system is faulting, and usually which sensor/part is the culprit, and then you can more accurately begin a diagnostic and repair effort. If the people at AutoZone cannot help you to your satisfaction, you can take the p-code(s) here and any one of our Techs can tell you what it means, and give you advice on how to diagnose and repair the problem.
Wether it needs immediate attention or not will depend on what the problem is. If it's simply an emission problem, no. If it's something that can effect engine performance, yes. You do need usually need to go to a Dealer as most local shops can deal with whatever problem there is, however, if you feel you'd rather be at a Dealer, any GM (Chevy, Buick, etc) Dealer can service a Saturn, as GM is/was the parent company of Saturn.