Deposits are in Settings, under Booking and payments. You can take a percentage of the service or a fixed amount, and it comes off the balance at the appointment.
Two things worth deciding before you switch them on. First, what happens when somebody cancels in good time — a deposit you keep regardless reads as a penalty, and word travels. Second, whether you want deposits on every service or only the long ones, because a deposit on a twenty-minute line-up costs you more in lost bookings than it saves.
Whatever you choose, write it in plain language on your booking page. Clients accept a policy they read before booking and resent one they discover afterwards.