A few facts to be known about gas/petrol stations in France :
Prices France, petrol, gas, summer 2021: expect about 1.50€ per liter. Cheaper will be Diesel, then SP95, then SP98. Cheaper will be supermarkets, most expensive will be highway service stations.
But by expecting about 1.5€ per liter you won't be far from the truth.
You can buy unleaded
(sans plomb) in either 95 or 98 octane, or diesel. Ethanol can also be found if your car is equipped.
Diesel is less expensive than unleaded petrol (gas) in France.
Both are a tad cheaper than in the UK, a whole lot more expensive than in the US.
Gas stations on the highways are usually the most expensive.
Gas stations in supermarkets are usually the less expensive.
Unless a sign says otherwise, you fill up your tank then pay.
All stations usually have clean toilets and a small shop.
Highway stations are attended 24 hours a day.
Other stations may be closed at night, or in automatic mode : no cashier, you pay by credit card (not all foreign cards accepted though).
If you run out of gas on the highway, no worries. Stop on the right hard shoulder, and either call 112 for help or walk to the nearest emergency phone (one every 2 kilometers), gas will be brought to you, but at a special price (gas price plus about 50£ or 80 US$).
Enjoy your driving!