My favorite is Loco2.com which is an English site so it is entirely in English, will give you prices in dollars and doesn't mark up the ticket prices. It has worked for me in putting together routes when the official sites failed - especially Renfe in Spain - and will list all the pricing options under the pink triangle. There are others but this is my - and many other people's - go to site.
Train tickets are generally available 60-90 days ahead so if you are planning for out and don't see them don't panic. Check an earlier date with the same day for ideas of what it will cost and the schedule. They usually don't change that much so a Thursday a month earlier than your actual trip will probably be pretty close.
All TGV trains will have seat reservations and be for a specific time and from station to station. Check the prices for flexibility options depending on how sure you are of your schedules and weigh the options. Tickets for local French trains - labelled TER usually - will be for a station to station direction for a certain day and will work for all TER trains traveling between those stations on that day. The one you book will be on the ticket in small print but you are not bound to that train if you get there early or late.
When you book smaller, shorter trains they will often not be listed on the schedule on the station monitors but instead the big cities will be shown - eg Marseilles will be the actual destination shown. Check the train number on your ticket to check the schedule. As the train gets closer to arrival time the smaller in-between cities will be listed as stops. Also be at the platform and ready at your train time - the trains blow into and out of smaller stations in minutes so be ready to get either on or off when it's time. It makes perfect sense but you need to adjust after riding the TGV to the local train sensibility.
Also many stations now feature electronic sign boards on the platform showing you where the train will stop on the platform and sometimes even which seats or classes stop where so you can be waiting at the right place when the cars show up. It's super helpful but was new to me on my most recent visit.
The man in seat 61 is a great informational site about trains in Europe with specific focus on particular countries: https://www.seat61.com/France.htm
Have a great trip!
=Tod