Yes, the only way to get to CDG is through the RER B, which connects at Paris Nord and Chatelet, both of which are on multiple metro lines, especially Chatelet which is essentially one of the "hubs" of the Paris metro.
If you are flying Air France, they do have "les Cars Air France" which is basically a shuttle bus to the airport. I'd recommend that, but if you can't, leave at least an hour and a half extra for the commute.
DO Not take a taxi, they are way too expensive in Paris