Travel on French Railways

Bucklander

(Moved from Introductions)

We are visiting Paris this year, staying to the South East of Paris on a site booked through the Club, then looking to use the local railway to get to the centre of Paris. My questions are do you get tickets on the train, at the station, or on line? Are there discounts for families? How do I get a timetable? We just do not know and it will be a totally new experience for us in France!
If anyone has used the French local train network particularly near Paris we would be interested in you experiences.
Thanks