As others have said, RER from CDG would be the simplest. There was some sort of problem with the train as I was leaving, and everyone had to get off and take a bus. The bus trip was MUCH slower. Fortunately I had plenty of time, but lots of folks were stressing over missing their flights. I think this is pretty rare, but it never pays to cut it too fine.
Cash excepting kiosks are not available at all stations, and they might be out of order. Your American credit card will not work at any automated payment location, and many merchants will look askance at it…but I never had one actually refuse it. At one point I had to beg a local to use his smart card to buy me a metro ticket, and let me pay him cash. As I was leaving, I noted that you can get some sort of (prepaid ??) temporary “smart” credit card at some of the currency exchange booths. Not sure how that works exactly, but it would be handy for the Metro, RER, and renting bicycles. Paris has what looks to be an outstanding bike share program. I say “looks” because without a smart card, I was driving my shoes everywhere the Metro didn’t take me.