RER is the best bet.
re metro tickets unfortunaly unlike London Oyster you can not get a Cart purple unless you are a paris resident – shame that really as having one also speeds you booking out a velib..
so re metro, you can buy books of tickets (multiples of 10) quite cheapley..
Not sure why people fuss about the metro in paris, easy to use and quick plus simple colours to follow – though it helps if you make a note of the lines final destinations as this helps with direction info!
also make use of velibs as well apart from a few euro reg on your visa they are free to use for upto 30mins
also not that it helps here but you can get flights to Orly (south of paris, which is a littler and less nightmarish airport than CDG, though not as well served)