2002 Nissan Altima catalytic converter

I bought my car about a year ago and 6 months ago I had to have my engine replaced it was under warranty so that wasn't so bad but now my check engine light came on and I need to get emissions to renew my tag so I had it hooked up and they said it was the catalytic converter so I started to look around at prices and found out there are 2 a front and rear how do I know which one to replace? I read about other people with my year and model with the same problems, was it my catalytic converter that put the stuff in my cylinders and tore up the engine, if thats the case is it messing up my new engine? Do I also need to replace the o2 sensor when I change the cat?