Different TGV's have different rules for bikes. On some, including the Paris (gare Montparnasse) to Saint-Nazaire route, you can reserve space for a bike for 10 euros per bike on many of the TGV's. On other TGV routes in France, there is no option to bring a standard non-folding bike. The number of bikes is limited, however, on TGV's which still have reserved bike space. Search for trains on the following website (do
NOT use the RailEurope website):
https://en.oui.sncf/en/train-ticket
As gerryl wrote, there should be a little bike logo on the lower left. Click on that, and you should see 3 choices. You probably want the following choice:
"1 non folded or disassembled bike (Not available for 1st class travel)"
Trains which have this bicycle option will be shown when you search for trains. The fee is presently 10 euros per bike on a TGV. If you wait until the day of departure, you stand a good likelihood of there being no bike space available. When 3 of us wanted to take a TGV from Bordeaux to Paris 18 months ago, reservations were full for bikes on the train that we wanted, even a day or two in advance. So we removed our pedals and folded our bikes and put them in large garbage bags that we brought for exactly that situation. That is similar to what gauvins described, but with our folding Bike Friday(s), we were actually able to put them in the designated baggage area since they were much more compact than a non-folding bike.