You'll be just fine. :) What a wonderful opportunity for you! I lived in France a few years ago, although I was there alone as a student. I made friends easily, and no one cared about my accent. I, too, had an "awful" American accent when I spoke French, but I found that many of the French people found it endearing. They really didn't care; they were delighted that I spoke French. Don't worry about popularity; instead, focus on being a nice and respectful person. Just don't go on about how the USA is better, although I don't think you'll have that problem. The French are very friendly and helpful, but they are not initially as "in your face" as Americans are. Many Americans like to smile and say hi to strangers; that is not the norm in France. Again, don't take that as coldness. In regard to fashion, I suggest you dress as you normally do, then gradually adapt your wardrobe after you have been living in France for a while. That's what I did, and I found it worked quite well. I don't think you'll find the way the French dress to be all that different, but it's the way they wear the clothes that impressed me. What I learned from French fashion: simplicity can be sexy and classy at the same time. There's no need to wear anything too garish - a nice pair of jeans, a pretty shirt, and a scarf can go quite well together. :)
Best of luck to you! You'll have a wonderful time! Many Americans would love the opportunity to live long term in France.