Lexus ES300 - P0135

I have a 1998 Lexus ES300 and the check engine light came on. I had it scanned by a backyard mechanic and the error is P0135 oxygen sensor -bank 1 sensor 1. Unfortunately he could not tell me where in the car it is installed.
I would like to replace it myself but I do not know which one to remove and replace?. Can anyone tell me how many banks and how many sensors are there and their location in this model.
Thank you for your help.

I see your post was in January. If on the off chance you still need a reply bank 1 sensor 1 will be on the firewall side on the exhaust manifold. Bank 1 is always bank with cyl. #1 and sensor 1 is first sensor.
Trev.

it's on the downpipe (closest one to the engine) measure between B+ and HT and you should get .8-1.4ohms @68F and 1.8-3.2ohms @1472;engine hot.