You'll be prompted to set this up when you first activate bookings, and can edit it at any time by going to Settings > Bookings > Personalise your booking pages.
This message will apply to all bookings on all of your classes as the Default Message.
You can choose to over-ride this message for any class in your timetable, from the Class Profile page. Simply choose your class from your Weekly Timetable list to view its Profile, and select the 'Confirmation Email' options on the right hand side of the screen.
This will open up a new screen where you can choose to swap from using the Default confirmation message to a new custom email message: