From Paris take the RER A Towards "Marne-La Vallee/Parcs Disneyland". It's 6,30 Euros One Way Ticket. Remember to save your ticket stub because unlike the regular Metro trains the RER trains require you to scan the ticket upon exiting. (Think NYC Subway versus Washington DC Metro). As stated before, the trip takes about 45 minutes dependent on whether it makes all those stops at the pesky "Noisy" stations.
From subway lines transfer to the RER A at Chatelet/Les Halles, Gare de Lyon, or Nation stations.
If traveling directly from Charles de Gaulle airport you can catch the TGV train which arrives at Disney in a mere 12 minutes. It's a pricier option though as tickets can go as high as $40. I do not recommend taking the RER from CDG to DLP. I did so on my last trip and it took close to 2 hours arrive at DLP.
If you have never been to DLP it's definitely worth your time to check it out. Have a great trip!