Which Paris airport? RER trains cover the Paris region only. La Rochelle is a common name including streets around Paris.
If you are landing at Charles de Gaulle airport and going to the town north of Bordeaux, the national rail company SNCF has TGV trains, the fast ones, departing from the airport SNCF station every couple of hours. There is one change of train and the journey takes about 4hr20, according to the SNCF website.
The trip on a train from Paris itself takes one hour less and does not require a change of trains. However, you have to make your way from the airport to the Montparnasse station which will probably require changing from commuter train to the Metro, taking at least an hour and adding to the cost. A bus from Air France Cars to Montparnasse is more comfortable but will cost more than the RER/Metro, and can get caught in traffic. At Charles de Gaulle the RER and SNCF stations are adjacent to each other in part of Terminal 2.
You will be a more confident and comfortable traveller if you research these itineraries yourself.
Here are some tools, not the only ones:
http://en.voyages-sncf.com/en/ The French inter-city rail system.
www.seat61.com The encyclopaedia of rail travel.
The terminal maps are helpful.
http://www.ratp.fr/en/ Interactive guide to Paris subways, buses, RER and streetcars.
www.parisbytrain.com Guided tours of some Paris stations with photos.
Hints: SNCF fares on main lines are cheaper when bought well in advance and will include seat reservations when required. If you live in the US/Canada choose Antarctica as SNCF ticket pick-up. You will be able to either print out your tickets at home or retrieve them from any SNCF station or boutique. Eurail can cost you more more and give fewer choices.