2001 Toyota Avalon O2 Sensor Problem?

Hi guys/gals. I've got a 2001 Avalon with 70k miles on it- The check engine light came on and the local ma/pa garage told me it was the O2 sensor. Very reputable shop, so I do trust them. They want $490 to replace it. I've got a few questions:

aa) I'm mechanically inclined and enjoy wrenching on domestic sport cars.....I've never owned a foreign car until recently- Can't I just change the sensor myself?

bb) How many 02 sensors are there? I was told 2? I'm assuming they'll be located under the car before the converter?

cc) Anyone know where I can get "online" instructions?

Anything you could tell me would be a huge help.

Thanks in advance-

-Busa196-