Do something like
Querida ( name of host family ) ,
thank them for allowing you to stay with them and try to picture a face to face conversation about you sharing what the question is asking about then talk about how you which to help others and ask about the organization also how you hope they don’t treat you as a guest but as a someone that’s there to help them as much as they helping you that’s where you put the chores you can help them with and finish with your date of arrival
finish it with
Cordialmente if you want something formal but if you want something more personal then un beso a todos ,
(your name )
