Roissy Bus is getting $$ to Opera. I'd do combo RER and Metro. The signage is excellent for transport--and just learn a simple, s'il vous plait, pardon and basics and you'll get assistance. Keep a good, simple map with you to show if you need help getting unlost. I don't favor Paris nor Parisiennes but only had issue a few times--most were very favorable and pleasant. There are plenty of sights to see. Take an umbrella. It can be a very damp grey city.

You can fill in local banks with tripadvisor.com. Good advice if you sift thru it.