Japan!!!

Along the lines of what Kana said, if you tend to be loud when speaking, laughing, etc. or tend to speak your mind impulsively. Learn to tone down the volume, quite a bit and think three times before speaking out about anything.

One thing I've learned about Japanese culture is that it prides itself on the respect of others. Being loud and boisterous in most situations is frowned upon as childish and immature and otherwise is irritating.

This is especially the case in social situations that are not necessarily casual settings. Riding on the train, being out in the public, does not imply a social situation. Even here, politeness and generally being quiet is expected.

Most Japanese people in the larger cities know that many Americans tend to be loud and boisterous, so they look past that most of the time.

But if you really want to immerse yourself in the culture, especially, if you plan to venture out from the big cities, I recommend that as a start.