I was a foreign exchange student in Austria for a year when I was 15-16, and I lived with two host families (everything went wrong with one so I switched to another), so this is a topic close to my heart. I've known and worked with well over a hundred other exchange students too. Where is the girl from and how old is she? How much English does she speak? I have so much I could type about here, I'll just stick to the basics. Please let me know how things go with her though and I'll try to help! This is advice for your whole family, especially your parents- have them read this post too, it may help!
-She might be tired and want to sleep a lot for a month or so after she arrives. This doesn't mean she's lazy, being in a new country by yourself surrounded by strange things and a new language is very tiring for the brain and it's normal to need more sleep.
-Absolutely do not jump to conclusions about anything she may say or do. Remember that she's from a different culture. If you don't understand something she said or are insulted or anything, please kindly ask her to explain what she meant. Same thing goes for you and your family, if she seems upset by something you say or do, explain what you said.
-Treat her lovingly. Tell her often that you're glad she's there, give her hugs if she likes hugs.
-Clearly explain all the expectations and rules your family has for her. Don't expect her to know to do certain things. If she doesn't do something (or does something you don't like), kindly explain it to her.
-It's normal sometimes for the other kids in the family to become jealous of the exchange student, especially if the parents praise them a lot. The parents should still praise the exchange student (and their own kids, of course!), but remember that the student is probably on her best behavior for the year she's there and it will be very distressing to her if the other kids act jealous towards her because she's being "good".
-Don't treat her like a guest. Treat her like a member of the family, with chores and all. She's going to be there for a whole year, the family will probably get tired of treating her extra special after a while and may come to resent her.
-Let her call home to her family sometimes. She shouldn't call her family every day, but every month or even every week for a while is okay. I have very vivid memories of my host father standing next to me the entire time for my once monthly 8 minute phone call home, tapping his watch, ready to hang up the phone at 8 minutes. My parents are divorced, so I had to choose which parent I wanted to talk to that month!
-Let her have some time to herself. It's not good if she spends all day locked in her bedroom, but it's okay if she wants to be alone sometimes.
I think that's okay advice to start with! What do you think? Did you have your parents read it? I don't know how well the exchange organization prepared your parents and family, I know mine sent a huge book to the first family to read, but they never did because it was "too long". I found it after I'd been living there a couple months and was surprised and relieved to find out that a lot of the things I'd been feeling (the tiredness, for example) were normal and not things I should have been punished for.