O2 sensors are in your exhaust pipes. Bank 1 sensor 1 would be the driver side in front of the catalytic converter Sensor 2 is behind the converter and bank 2 is passenger side. They run around $45 and are usually easy to change. On my 2005 Avalanche i swapped out the front driver side sensor right before I sold it. The Tahoe should be the same. Getting my arm up in there to get it out was a real pain in the butt. If you have skinny arms, it should be easy. You can either use a wrench to bust it loose or get the socket with the slot cut in it form an auto parts store. I just used a wrench. Once it is unbolted, the hard part is getting it unplugged. There is a small wire tie locking the plug on, it has to be cut off.
Also, the parts store guy said a faulty gas cap seal could also make that same code pop up. You can try that first for like 12 bucks. I swapped out the gas cap and it didn't get it, but the new sensor fixed it.
Good Luck, with a little patience or long skinny arms, its fairly easy.
