A few more notes about trains in France:
From PACA (Provence-Alpes-Cote-d'Azur) you will find two types of trains: the TER and TGV. We don't really have the Intercités here.
TER are the regional trains. Take these between towns, e.g., from Nîmes to Arles, or Avignon to Carpentras. For short trips between the small towns, you don't need to buy a ticket in advance - the price will be the same. The dates of your trip are not during peak holiday season, so the trains will definitely have space (you do not need to worry about advanced booking for a TER). You can buy the ticket from the kiosk in the station, from a desk in the station (if there is one), or download the SNCF app on your phone (if you'll have a data plan).
TGV are the "trains à grandes vitesse", the high-speed trains. These trains travel up to 300 km/h, and do not make frequent stops. You will have an assigned seat on the TGV, in 1st or 2nd class (unlike the TER). For the TGV, book your tickets ahead -- not only do prices go up closer to the travel date, but they can sellout.
There are two types of TGV: OUIGO and INOUI (or TGV). INOUI is the normal TGV, and OUIGO is the "cheap" TGV. They are both SNCF. The OUIGO goes to fewer stations, requires you to be at the voie (platform) 30 minutes before departure, and will charge for excess luggage (i.e., they are trying to adopt a model based on Euro budget airlines), and tickets can be very cheap. Both types of tickets are sold on the SNCF website, or if you want to look only for OUIGO tickets, they are at OUIGO.com.
To travel from Nîmes or Avignon to CDG, you will want to take a TGV. As someone said, there is a direct train from Avignon TGV to CDG (3h15). From Avignon, there are multiple TGV lines that go to Paris: one that makes many stops (Valence, Lyon, ...) - and this one is slower. There is also a direct TGV from the new Nîmes Pont du Gard station to CDG (3h30) - but note that this station is actually outside of Nîmes.
I hope this helps!