If you have a TGV ticket you necessarily already have a seat reservation as these are compulsory for TGVs. It should say something like "voiture (coach) XX", "place assise (seat) YYFENETRE/COULOIR" (Window/aisle), and in the case of a double-decker "DUPLEX: EN Haut/Bas" (Upper/lower level).
As for domestic TGV, the only perks in 1st class are the lower density seating (2+1 instead of 2+2), the higher seat pitch and power sockets. No meals or refreshements are included (you can purchase these in the buffet car at highly inflated prices) and neither is WiFi...