Basically, the light is just telling you that something isn't quite right. It may be as simple as an exhaust sensor that had a hissy fit and gave off weird values, and is now back to normal, or it could be telling you there's a misfire, etc. Usually it's something really simple, but the code will tell you exactly what's going on - so no need to panic.
I'm in the UK. Your first option is to get a garage to do it. Halfords do a diagnostics check, but it's £50. A lot of places will try to charge, but your best bet is to try and ring around and see if you can get it done for free.
The second option is the easiest. Buy a WiFi OBD2 adapter on Ebay (about £10) and download an OBD app to your phone and read the code yourself (OBD Fusion for iOS, or Torque for Android) The adapter plugs into the diagnostics port of your car in the footwell, you connect your phones WiFi to the adapter (like you would to your internet in your house), launch the app and the code will pop up. You'll also have the ability to clear the app too. Really easy
When I bought the car, I had an error code telling me one of the lambda (oxygen) sensors on the exhaust was reading way too much air. I looked at the sensor just underneath the back of the car and it was loose - tightened it up, cleared the code myself with the app, and all has been fine since.
So yeah. Don't panic, get someone to read the code or do it yourself for cheap
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