The first difference you will notice in France (especially coming from a small town/school like Marietta) is that people do not smile at strangers. If you smile at someone you don't know, they will assume you are not from the area, and they tend to think you are strange. Our french professors told us that to look like we fit in we should look straight ahead while walking and maintain a more serious demeanor.
The next cultural difference to be aware of- always, ALWAYS, say "bonjour" when you enter a store or restaurant. This is a simple act that goes a long way to the French, and if you fail to say bonjour you may not be served, or you may encounter some rudeness. It seems unimportant to us, but its considered just having good manners in France.
Lastly, the French are very private people. When you are invited into a French person's home, you are a guest, and you should not "make yourself at home" even if they tell you to. Often when you enter a French household you will not be given a tour of the house, unlike in the U.S. The only rooms I have seen in my home stay are the living room, kitchen, and my bedroom/bathroom. It is also rude to clean your own dishes at someone's house; it can be seen as you thinking they are not good hosts, or that they are not maintaining a properly clean home. It is always polite to offer to help with cleaning, but you will usually be told to "reposez-vous," relax yourself.
NOTE: These of course are generalizations, and may not always be true in France or French cultures, but they are a good jumping-off point for understanding the French a little bit better.
The City of Water