You do not get to select which train station to use. Paris has several mainline train stations: Gare St-Lazare, Gare du Nord, Gare de l'Est, Gare de Lyon, Gare d'Austerlitz, Gare Montparnasse and Gare de Bercy. Each serves a different area (with some small overlap for Montparnasse and Gare d'Austerlitz and Gare de Lyon and Bercy) -- for example, trains to Normandy leave from Gare St-Lazare. If you are purchasing your ticket from sncf.com/en or CaptainTrain.com you select Paris as your departure point and enter your destination and when the results pop up, you will see which of the train station that will be your departure point.