Euro Disneyland has a train station right on the fair grounds, called Marne La Vallée--Chessy station. Here is a
photo guide on how to get from Paris to Euro Disney, so you'll need to follow these instructions in reverse.
Two different types of trains stop at this station, an RER A suburban commuter train and a TGV high speed intercity train. The TGV train travels to Charles de Gaulle Airport at Roissy. The RER A train travels through central Paris, leaving from Marne La Vallée--Chessy station every 15 minutes or so. This train will take about 40 minutes stopping at Paris stations: Vincennes, Nation,
Gare de Lyon, Chatelet Les Halles, Auber, Charles de Gaulle Etoile, La Défense (that's going east to west). The most central station of those would be Chatelet Les Halles, which is close to the Louvre. Auber is close to Galeries Lafayette and Opera in the 8th Arrondissement.
Check out the instructions linked above and let me know if you have some questions.
Ben