Create an event as described in "How to create an event".
To make it a virtual event, from the Design Event Campaign tab of your event, click the Landing Page tab -> go to the Overview section -> Virtual Event header. Check the box that says, ‘This is a Virtual Event’. This will set the campaign as a virtual event.
Once the event is set as virtual, the Venues tab will not be visible anymore and you will not need to fill in the physical venue details for the event.
There you go, your virtual event is ready and set to run!