To see the David of Michelangelo, you have to purchase the ticket for the Accademia Gallery.
If you want to book the visit (so that you can skip the queue) you can do so purchasing the tickets online on the B-Ticket website or the BeCulture one, otherwise you can contact the call center at +39 055 294883, active Monday to Friday from 8:30 am to 6:30 pm or Saturdays from 8:30 am to 6:00 pm.