Once logged in to your organiser account, on your dashboard go to the event you wish to amend.
Under Manage event on your menu, select Edit event details:
You have three options:
1) Set your event to go on sale at a certain date/time with a visible countdown timer. To do this, simply set a Tickets on/off sale date and time.
2) Hide your event until a certain date/time.
3) Set your event as Private for now, and then set to Public whenever you are ready to go on sale.