My best advice is to arrive at CDG, and have a hotel booked in Paris. After customs, follow the Paris by Train signs in the airport. When you get to the part where you go downstairs to the train station, follow the signs for isle de France (the local trains) not the grande lignes (TGV) station. Buy a ticket from a machine for the RER line. You need to have Euros, your credit card probably won't work. The RER B is the only train, but each one will have different end points. It doesn't matter which one you catch, they all go where you are going. The train goes right through central Paris. If your hotel is within a few blocks of the Montparnasse station (it should be), get off at Denfert-Rochereau station, and walk to your hotel from there.
Download the directions from the metro station to your hotel before you go so you can find your way before your phone is working. Nearby, you can find a phone store, and pick up a SIM card for your phone. Rehydrate, walk around and sightsee, eat something, and hit the sack.
Buy your TGV ticket online before you go. Rail Europe and trainline are good sites for getting TGV tickets. You book a specific seat on these. There is a train to Bayonne from Montparnasse every couple of hours, I catch one at 10 the next morning. It's under 4 hours total to Bayonne. You will see people with packs boarding the same train, and all will get off in Bayonne. In Bayonne, change to a local TER train for sjpp. You can buy that ticket when you buy your TGV ticket, but you don't have to. The local trains can be bought at the station, and you just find a seat. From sjpp, walk to your hostel, or whatever. Sjpp is a small village.