Former high school exchange student here. I spent a year in a country and didn't have a real opportunity to learn the language until I was there. I know this isn't something most people can do, but in general I think a key is immersing yourself in the language as much as possible. Do more listening than talking at first. If you keep hearing a word or phrase come up over and over, look it up and write it down. Chances are if you hear others using it a lot, you will need to as well. Make a good translation dictionary your friend.

I would also suggest immersion in as many realms of expression of language as possible -- speaking, writing, reading and listening. Podcasts and TV shows are great ideas! I would also recommend reading subreddits devoted to aspects of French culture or politics. (Don't forget there are many countries outside of France where French is primarily spoken, too!) I've heard a lot of good things about apps like Duolingo for practicing speaking and building vocabulary.

The biggest advice I can give is don't be afraid to take risks when you start conversing with native French speakers. I noticed my language skills skyrocketed after I had spent the first two months speaking very little and finally started to open up more to my peers and host family. I still don't speak perfectly even after 20 years, but the more confidence you can build in your skills early on, the easier it will be to be comfortable communicating. Good luck!