You should definitely go. Living in a foreign country away from people you know is a good way for you to grow and develop as an individual. I did an exchange program for the first time during the summer of my senior year, and it really set me straight. I'm actually living and going to a university in Japan now just because of finding my true love from doing a home stay in Japan.
Anyways, on to the list
Sense you are staying with a host family, always be on your best behavior. They may be "family" and tell you relax, be your self and help your self to the fridge, but still be polite and act as a guest. Don't take advantage of everything they tell you can do, make sure to always ask if something is ok, even if they said it was ok. Japanese people don't tend to be so open to people right away like Americans do. Example, don't go out and mention that your parents are divorced the first day you are there. This is something considered kind of personal and private, so they may be shocked if you mention something like that.
Be friendly and polite to everyone if possible. If you get homesick, try not be on your own all the time and lock yourself in your room. They will worry and it may be best to explain to them what is wrong and such. Also as an example of manners, if you have to do something with people or have any food, always offer it to them first. Also if you are served first at the table, either wait for everyone else or ask if you can eat first.
If you make friends and plan to do things with them, always tell your host family in advance. Your host family might plan stuff ahead for you to do with them, so don't just call and tell them your not coming home for dinner, plan it at least a day in advance.
Closeness to friends might not happen. Again, Japanese people are not like Americans and won't think of you as their best friend in a day. For me just at my college, it took me about a year to get to a level of closeness with friends to talk about family and such issues.
If you speak any Japanese to the family or friends etc, it is always impolite to give a direct "No". Japanese people never tend to say no to you right away, they tend to swerve around it and make it sound more like a yes. For example, if you ask if you can go out that night, they might reply as in "well, I was planning on making a pie tonight" (well it might not be exactly like that, but expect something indirect that usually won't contain the word no".
Also if your an anime freak, like the majority of foreigners that go to Japan, don't just blurt it out to everyone. Not everyone likes anime here, and might think of you as one of the stereotypical Americans that come to Japan, just because of anime and nothing else.... Also expect your host family to ask if you have a gun or if all Americans carry a gun. I was actually shocked when my host family asked me that questions, and I'm still shocked today when some people at my school ask me that question. sweatdrop
Also, just as a last note.... don't plan to buy a computer here... I just bought a dell here and I'm having a hard time adjusting to it. xp