This is an amazing experience. I have had a similar adventure, it can lead to an amazing experience and a meaningful friendship. I don't know which country you are going to, I have visited friends in Mexico, Guatemala and Jordan (the latter had the most cultural differences). But here are some hopefully helpful comments.
1. Be a decent human being. Think about what you will most want from a guest when you are in their shoes. Be gracious, appreciative, respectful... I think you got this.
2. Respect cultural difference. In Latin American countries people are willing to joke around in ways that would offend many Americans. In Jordan, single men are separated from women and families (I pretended to be temporarily married to one of my friends relatives so I could get around this). Appreciate the differences but realize that this is their country and their ideas about what is appropriate are the only ones that matter.
3. Set your own boundaries clearly and respectfully. If there is something that you are not comfortable doing, simply say so respectfully. Again, put yourself in their shoes... if your international visitor said they respectfully that they weren't comfortable doing something you would respect it. What is important is that you aren't judgemental; but you can say as a personal matter that you don't want to do something.
4. On the other hand, be very open minded. I have eaten crickets. I have been naked in a traditional steam room. I have paid for a dance in a smoky traditional dance hall (just a dance). I have learned to kiss people on the cheek in a social setting. I find that these experiences are some of the best in life.
5. See this as an incredible adventure. You will get the chance to view the world from outside of your own culture, and you will see your own culture better. We all take our own normal life for granted, until we understand that other people see things differently and are just as decent and happy.
6. I want to comment on your statement " I *desperately* need this trip to go well.
. I think this is a bad way to look at it. There are no guarantees in life and there is a possibility that this trip will not go well.
I hope that you can get over this attitude and the fear that goes around it. It you are desperately trying to make things go perfectly, you will spend all of your time nervous and you may miss out on the best parts of the experience.
The worst thing that can happen is that you won't get along. In that case you will make the best of it. You can always get a hotel (I can't imagine it going that badly). And there are more people and more adventures to be had.
I fully expect that you will have an amazingly good time. But relax... be respectful, be decent, be open minded. You are putting yourself in a great position to have an adventure ... embrace it!
I wish you the best of experiences.