Not sure which button to click?
If you love the space and are ready to book, click on Request to Book (your card won't be charged until the host accepts the booking).
If you have a few initial questions about the space (ie. clarifications about certain amenities, or wondering if your booking is appropriate for the space), click on Ask a Question. When you are ready to confirm the booking, let the host know and they can send you an offer within the same message thread.
When messaging the host, no matter if it's a booking request or a simple question, make sure to provide as many details as possible about what you intend to use the space for. The host needs to make sure your request is suitable for their space.