When is comes to French food, it is delicious, but very different from any other food. I recommend that before coming to France, go to your pharmacy and buy some pills called 'probiotics'. These pills will help line your intestines with extra cells so that your body can cope with the change in food. Otherwise the food change can shock your digestive system and you'll get sick (diarrhea, vomiting, just grossness in general). So buy these pills and take them once a day for the first week before you leave for France, 2-3 times a day for the first week after your arrival (3 times is preferred, but 2 times will probably work as well), and then just once a day for the next two weeks afterwards. This way your stomach is eased into the transition, and you won't get sick.
Now some people have strong stomachs and the change won't effect them at all. I'm sure there are people reading this and saying 'I went to France and I never got sick!' But hey, everyone is different. And for some people the change will be a bit delayed. My last trip to France when I was 16 I didn't get sick for the first three weeks, but week four came around and I was miserable! So just stay on the safe side, get probiotics, and stay healthy!!