Has anyone taken a train within France we wanted to take a ride down to southern France for a day anyone know any website for the fares or give me some of there experiences with the service.
Rafael,
Look at the big picture first. You said that you wanted to ride down to southern France for a day. I have no idea of your definition of southern France, but Marseille is about as far south as you can get and I'm sure it's served by high-speed trains from Paris.
I'm not a train person, but I'd bet that such a trip would take a good three hours. Figure four each way by the time you get from where you're staying to the departure station and the time from the arrival station to whatever you want to see. That's not a lot of time left over for looking around. That first link in the post above should give you an idea of the cost so you can decide if it's worth it. The second link looks like it puts you in the wrong part of Seat61, try this one instead: http://www.seat61.com/France-trains.htm
I would agree with Ed that if you are staying in Paris (as I assume) the real South of France is too far for a day trip. Paris to Marseille is three-and-a-half hours, and you should also consider what would happen if trains were badly delayed.