How can I travel by train from Aix en Provence to Boudreaux then to Paris?
I'd recommend using SNCF-Connect to book your France intercity train tickets. This is the state owned/operated rail company ticketing website.
For a guide on how to purchase tickets using their website, see this page on buying TGV tickets for CDG to Disneyland as an example, but change the origin / destination to suit your needs (i.e. Aix-en-Provence to Dijon, Dijon to Paris - Gare de Lyon).
Be aware of the difference in trains & speeds offered. TER trains are "regular speed" and TGV are "high speed" trains between your destinations.