Hey Jenn -
Here you will find tons of former students who have studied in Spain like myself, taravb, granada girl, Monica and a host of others (Puna?)...
My advice:
1. Don´t worry about your family understanding or not understanding you. Chances are they are in the hosting business and you won´t be the first (or worst speaking) student they have ever had.
2. Your host family is looking forward to meeting you. Afterall, they have opened their doors to you! Any kind of communication from you prior to your trip will most likely be appreciated.
3. Spain is a modern country like the USA. The added benefit is that such modernism is mixed in with some of the most awesome historic sites in the world (historic...I mean way older than the USA itself).
4. Smile! You are leaving chilly Illinois and coming to Madrid (I think) where the temps have been in the 70s lately.
5. Really GRIN! You are about to embark upon one of the most life changing experiences you will ever have. Unless you are in a minor few, you will love, I mean LOVE every minute of your adventure!
Good luck and have fun!