Even for seasoned travelers, the Metro can be a little confusing. The Paris Metro app was really helpful. You put in your Metro stop, your destination, and the app shows you exactly how to get there in a very clear way. Highly recommended!
I usually use Google Maps to navigate public transit in Europe. In Paris, it will tell you how to get from place to place by walking or public transportation, including Metro, RER, or bus. Sometimes it will show you a direct bus to your destination instead of taking say two Metro trains.
The only problem I had with Google Maps in Paris last year is that it seemed unaware of construction on the Metro and RER lines, and sometimes it gave me bad directions (the train is not running past a certain stop due to construction; so when you get off, Google Maps will just tell you to get back on). I understand that other apps like City Mapper are better informed about such things, but I haven't tried them. Google Maps has worked great in other cities, though, e.g. navigating the vaporetto boats in Venice.