We went to Paris by train today. The ride takes 35-50 minutes (depending on the train station). You need to know where the train stations are, and what ticket to buy.
The destination in Paris is Gare De Lyon station. The first station is Thomery. 4 minutes later is Fontainebleau-Avon station. In 7 minutes you will be in Bois Le Roi. From there in 35 minutes you are in Paris (through Melun).
The way back is the pretty much the same.
A single two-ways ticket to Paris – Gare De Lyon station – cost 15.20 Euros. Order a ticket of 15.90 Euros and you get a free Metro pass for the entire day! This is really worthwhile. Don’t forget to write your name and the date on the ticket. Also stamp the ticket in a machine that is located on the deck.
From Gare De Lyon station runs Line 1 of the metro, which takes you through Champs D'elise to La Defance. (When you come back to the station go up 1 or two floors to the train station). There is tourist information in the station, where you can get a free map. You can ask around the station, people are mostly friendly and know English surprisingly well.