The OP has planned stopso ly in large cities; is there any reason that you've selected Marseille, for example, rather than, say, Arles or Aix-en-Provence? You might read a very recent discussion on LP about Marseille!
As others have said, SNCF is, for intercity travel,the only rail company you'll need to deal with.
If you find the site called "SNCF voyages" you can find schedules and prices, and buy tickets online. You'll be asked about your country mainly because you'll need to specify where you'll take possession of the physical tickets. You can say "France", and then you'll receive a code with which, along with your name and credit card used for the purchase,you can receive the tickets at any SNCF office in France. In some train stations, the lines at ticket windows can be very long, so don't plan to get your tickets at the last moment before departure!
TGV means high-speed train, and you'll absolutely need a reserved seat on those trains ( for example, between Paris and Bordeaux). For other trains ( depending on which day, and season of the year, seat reservations may not be needed, so you can achieve more flexibility in your travel plans.). If your planned travel will be in July or August, it'll be very important to reserve a seat, and less important during the Autumn.