Where is sensor 1, bank 1, located on my car?

You said "first"! How many 02 sensors does an 2003 Impala, 3.4 engine, have? And, if one bank needs replacing, is it "normal", or "practical" to replace all?

You can change them both if you wish. You have two on this vehicle. One is in front of the catalytic converter and the other is behind it toward the muffler. The code you have if for the one in front of the catalytic converter.

Okay! Now, once I replace them, how do I reset the "check engine" light, or get rid of it? Or does it go away "automatically" once they are replaced?

After the computer in the vehicle has completed a full diagnostic cycle (which is different in all cars) the light should go off. Just drive normally and in about 150 miles it should reset. You can take the battery wires off wait 15min to 20 min and the computer will reset and the light will go off. You will still have to complete the diagnostic cycle to be sure it does not return.