Train Travelling in France Hi

I am going to visit Royan for four weeks, to learn French. As I am coming from Germany and Royan is in the west , I need to travel all the way through France. As I am short of money, I am looking for the cheapest way to do so.
In Germany it is possible to buy a single ticket which allows you to travel one day with any short-distance train you like. This takes a lot of time, but is kind of cheap.
Is there something similar in France? And how can I find a route via these kinds of trains?

Greetings,
Peter