The RER (Réseau Express Régional) is a network of express trains that run from central Paris to its suburbs. There are 16 metro lines and 5 RER lines that run throughout Paris. If both a metro and RER station are equally close, the RER train will likely be faster as it is an express train.  Search alternate train options; depending on your starting location there may be a faster train. For example, there is a TGV (Train à Grande Vitesse, or high-speed train) direct from the Charles-de-Gaulle Airport to Disneyland Paris.  There are also Eurostar trains to Disneyland Paris direct from several places in the United Kingdom. You can purchase these tickets from any metro or RER station ticket window, or from any of the automated ticket vending machines located around Paris. If you're traveling from central Paris (public transportation Zone 1) this is the only ticket you'll need to purchase.  A one-way ticket from central Paris will cost less than 8 EUR (as of August 2018). A ticket t+ will NOT work for this trip since your final stop is in Zone 5, do not be a cheapskate. A fine of 35 EUR will be given if you have a wrong ticket for the wrong zone. Pass Navigo Decouverte is accepted. Paris Visite Cards and Ticket Mobilis are accepted IF AND ONLY IF Zone 5 is included. Ticket Jeune Weekend is accepted IF AND ONLY IF Zone 5 is included, AND you are under 26 and travel on weekends OR bank holidays. If the closest RER station to you does not serve the A train, you will need to take a different RER or Metro line and switch over to the RER A train. For example, if you're traveling from Charles de Gaulle airport, take the RER B train to Paris and switch to the RER A train line towards Marne la Valleé at Châtelet Les Halles station. The sign for your direction may also read "Boissy-St-Legér", in addition to "Marne-la-Valleé". As you're waiting for your train, make sure that the stop panels hanging above the train platform have a yellow box next to the Marne-la-Valleé - Chessy station.  If Marne la Valleé - Chessy is not listed on the hanging panels, you're on the wrong side of the platform. Keep your ticket handy while you're aboard the RER in case you need to show it to a ticket checker (known as "controllers"). The Marne-la-Valleé/Chessy stop is a two minute walk from the park gates. Exit the RER A train station and go up the escalator, following the signs for "Sortie". Use your billet Ile-de-France in the turnstiles as you exit.
Find the closest Metro or RER station. Purchase a Billet Ile-de-France. Make your way to the RER A train. Take the RER A train line towards Marne-la-Valleé - Chessy. Exit the station and walk to Disneyland Paris.