score:8
The Paris transport operator, RATP has a journey planner on their website. You can input your start point, end point, and start time, and it will suggest routes to you. If you don't want to worry about the bus, you can restrict it to rail (so Train, RER and Metro) only. Make sure you search for Aeroport or similar as your start point however, as France is full of places named after Charles De Gaulle. For your journey, the exact answer depends on the time of day you arrive, but taking the RER B to Châtelet-Les Halles and then the Metro line 7 towards Mairie d'Ivry looks the most straightforward. Châtelet is a big and confusing interchange however, so be ready for that.
Broadly speaking, there are several main components of the Paris transport system:
You can see Trams (dashed lines), RER (thick lines), Metro (thin lines) and mainline train services (double lines, linked) on the RATP online map (You can see CDG airport to the north-east). RATP also provide downloadable maps, and paper maps once you get to Paris.
There are a large amount of ticket options available. For the sake of your specific journey, you are probably best off buying a single ticket from the airport, to your destination, from a machine or office at the airport station. For future journeys, you may find a standard ticket, or a pass useful, but as CDG is a long way out, a pass that covers that journey to is probably bad value. You can read a full description of tickets (in French) here, including the "Billet Origine-Destination" that you probably need on the RATP website. There is an English language page, but it only describes some of the ticket types.
Upvote:2
The quickest way seems to be :
It should take about 1h15, price should be in the vicinity of 15€ (need confirmation).
Be aware that Châtelet-Les Halles is a pretty big station and that you might walk a bit.