By default, you will receive an automated email when a guest completes an online booking form.
What you then need to do will depend on how you have chosen to receive bookings.
If you accept Instant Bookings, the booking request will already have been confirmed to your new guest. If your website has been setup to receive Enquiries Only, then you need to make sure you are happy with the information provided by your prospective new Guest before confirming the booking through your website or getting in touch with them direct to arrange payment.