White Smoke and running rough , when it runs !

My 1995 Plymouth Voyager has white smoke billowing from the tailpipe first thing in the morning. There is also fuel coming out the tailpipe. The white smoke continues until the car is warmed up sufficiently , then stops. A friend told me it might be the O2 sensor telling the computer that the engine needs more fuel , therefore sending too much fuel , and the excess coming out the tailpipe. If he is correct , where do i find the O2 sensor on the motor