Never been to France, but in the USA it's really easy. The biggest thing is to remember what pump your car is at. Just walk up to the clerk and say, "$30 (or whatever) on pump three please." You can often pay without even walking into the store if you pay with a credit card. It's because you can just swipe your card at the pump. I bet it's the same or really close in France.

Bonjour It is close in fact (lol!) or you can say : " Bonjour, le plein s'il vous plait" if you want to full the tank.

What I found odd in Paris, was petrol stations in themselves. Just a pump between parking lots on the side of the street. Hard to see to say the least. No clerks obviously. I could not believe it, I took a photo. I don't know if it's all like that one I saw (I was not driving). In Canada, there is not much pump with service left. You fill the car and then pay inside. You'd say "pompe numéro..." at the till and the clerk tells you how much you have to pay.