From Gare du Nord
you first need to get to Gare de l'Est. You have 3 options: Metro Line 4 (direction Porte d'Orleans), Metro Line 5 (direction Place d'Italie), or on foot as the two stations are rather close to each other as visible here on Google Maps
From Gare de l'Est, take Metro Line 7 towards Villejuif-Louis Aragon/Marie d'Ivry until Opera station.
If you're coming from CDG you can also take a Roissybus which delivers you directly to Opera from the airport.