Transportation from CDG to Trocadero in Paris

You could take Air France bus #2 to Paris-Arc de Triomphe (Place de l’Etoile). There is one stop at Porte Maillot and the fare is €13. From there it is a short Metro (Line 6) or taxi ride to Trocadero.

Or you can take the RER B to Gare du Nord, walk through the tunnels to the La Chapelle Metro station and take Line 2 to Charles de Gaulle Etoile, and take Line 6 from there to Trocadero. The fare is €8.20.

See this zoomable map of the Metro and RER system for central Paris.

There are several ways to get to Trocadero the first option suggested by Tim to Charles de Gaulle Etoile would be the easiest -

Upon arrival with the Air France bus - you will have to purchase a metro ticket. Take the line 6 to Trocadero. Taxis may be a bit short (distance is not even 2 miles) and you might find lack of enthusiasm to take you.

I would not recommend the second suggestion with the walk to La Chapelle through Gare du Nord - it is long and with luggage and directions possibly confusing to a new arrival.

Should you be determined to ride the rails - you will have to make several changes - RER B to Chatelet (blue) change to RER A (red) to Charles de Gaulle to the Metro 6 to Trocadero. (Oddly easier than La Chapelle because the change at Chatelet is walking to the quai next to where you get off no stairs). It is one stop between Chatelet and Charles de Gaulle Etoile.

Jona's route (RER B to Chatelet-Les Halles, RER A to Charles de Gaulle Etoile, and Metro line 6 to Trocadero) is definitely better than the route I suggested. Follow the "Correspondance" (connections) signs.