Change the forward O2 sensor. It's pretty easy. The sensor is available at any auto parts store, and should be about $70-80. The job will take about 15 minutes. Autozone will loan you the special socket for free.
Look behind the engine where all the exhaust pipes come together and you will see the sensor. Unplug, unscrew, put in the new one, plug it in and drive.
You may have to drive for a while before the code goes away, or you could clear the code with your OBDII tool if it is capable of doing that.