Regardless of whether you will be participating in the full-year or one-semester JYM program, once you have been accepted for admission to JYM, it may still be 4-5 months before you arrive in Munich. Most likely you will not be taking a German language course at your home college or university during this "down time." And that runs the risk that your language skills may have started to turn a bit rusty by the time you get to Munich.
Without exception, every JYM student has stated that they wish they had studied more German before the program began. If you're a good student, you should challenge yourself to become better. If you're a very good student, you'll want to become even better.
Your ability to reach the goals you set for yourself in Munich depends on your ability to understand and communicate in German. There's no way around that. So don't put it off, and don't be put off. Afterall, learning a second language is life-long project and even your professors still need to devote time to keep on learning German.
Don't let anyone tell you that there are no cultural differences between Germany and the US (nor that everyone in Germany speaks English, for that matter!). There are many differences: some are big, some small, some subtle, some sublime. Although you may not entirely appreciate them by reading about them in advance of your arrival in Munich, by doing so now you'll be in a better position to recognize them when you see them in action (and even avoid a cultural faux pas in the process).
You may find this an odd bit of advice, but yes: make sure you know what's going on in the US before you arrive in Munich. Germans are generally well-read; they keep up with the latest news, and enjoy conversation and lively debate. Don't be surprised if they know more about what's happening in the US than you (although that would turn out to be quite embarassing, nicht wahr?). As much as you will want to learn as much as you can about Germans and life in Germany, so too will they want to learn about Americans and life in America. That's why we've included a number of books that compare Germany and the EU with the US in the reading list that we think you'll actually enjoy reading (and provide fodder for the beginnings of interesting discussion!).