sar5w,
Replacing the front or the rear O2 sensor is a fairly straight forward job. You must work from under the car to actually remove the sensor.. unless you have long arms. You can reach the end of the wiring harness from the engine compartment. The O2 sensor looks like a spark plug screwed into the exhaust pipe just to the rear of the engine. There is a special O2 sensor socket that can be purchased at Advance, Autozone Pep Boys etc. Although I have the special socket I have used an open/box end wrench to remove and replace the sensor. The process is simple. From under the car locate and remove the sensor with the special socket. Follow the wiring harness toward the front of the car until you reach the 4 pin plug. With your thumb or other strong finger press the wire locking device in and pull the plug out. Installation is the reverse of this process. You will need to place some anti-seize compound on the threads of the new sensor if the new sensor does not already have some of the anti-seize material.
There is great deal of information on this board dealing with O2 sensors. You would be well served to study some of these resources to get information on where to purchase yor sensor and other options that are available.
Another thing you need to check unless you are 100% sure that you have a bad sensor is the vacuum hoses on your engine. I too had the infoamous "Check Engine Light. I found a vacuum hose loose, reconnected it, reset the code and have not seen the "Check Engine Light" since and that was many miles ago.
Good luck!!