Self Diagnosis Scans
Lucky for you I have scanned the pages in my FSM detailing Self-Diagnosis Mode for the ECU. Follow the instructions and the car should give you a code telling you what the problem is. However, there are some parts that can't self-diagnose themselves, in this case, get thee to a shop.
IF it's your O2 sensor (which mine was about a month ago) do not take the car to the dealer for it. They'll charge you $300 or so dollars, and you can do it yourself for $70, 5 minutes, and a wrench.