This page outlines the ways students can prepare for their year in Paris in their personal lives. I've been accepted, now what? outlines the administrative process for students preparing to leave.
Building confidence using your French skills in day-to-day life is the single biggest way you can prepare to land in Paris with your feet on the ground. Even for students with extensive French skills from the classroom, suddenly facing rapid-fire French from native speakers 24/7 will likely be overwhelming. Here are some suggestions from a former JYA student for how to prepare:
Watching French TV shows with no subtitles or subtitles in French. The more familiar you can get with how young people speak to one another, the better. Even if you understand what the professor says in the classroom, young people will likely speak to one another in slang, unfamiliar grammatical structures, and converse with startling speed. While watching without closed captioning is better practice (people won't be speaking with subtitles), subtitles in French are useful in showing the written forms of unfamiliar words. Watching these kinds of shows will give you useful practice listening that will make it easier to participate in conversation when you encounter these situations in real life.
Reading contemporary French novels and listening to podcasts or news in French can serve a similar function.
If you live far from Northampton, packing for Paris may look similar to packing for Smith. Bringing fewer, more versatile/basic clothes is always a good idea, since storage space in your host family may be limited, and you will need to fit everything (plus whatever you buy in Paris) back in your suitcase to go home.
The weather in Paris is generally somewhat more mild than Northampton: winter temperatures are often above freezing, while Northampton winters are often below. Summers are a bit cooler as well, but consider when packing if you'll be in Paris during the summer after the school year ends.