You can force incoming bookings to be viewed as "requests only", or "provisional bookings", until accepted by you.
1) Go to "Settings -> Booking form -> General options for booking form -> Edit > Booking requests : All bookings are directly considered final".
2) In the drop down either choose "Bookings made online by guests are merely considered as requests until confirmed", or "All new bookings are merely considered as requests until confirmed" instead of the default value.
3) Now when a guest fill in the booking form on your web site the button will state "Send booking request" instead of "Book now". After clicking on the button the guest will receive a email with a booking request confirmation. Later on when you confirm the booking they will receive a final booking confirmation as well. After the booking request is made the guest get this message:
4) By clicking "Confirmed" in the top left corner in the bookings you confirm the booking and a confirmation email, and SMS if you have added that, is sent out to the guest. You can create your own request and confirmation email and SMS templates like this.