This simple issue about renders image and video is easy to solve.
Go to Properties Editor and select the Render.
As you know already, the Properties Editor is used to edit data and properties for the Active Scene and the Active Object.
First, check if in the Output tab (from same Properties Editor) the output image is RGBA.
Into the Render, tab area finds the Film and select from Sky to Transparent.
Use the key F12 to render your output.
The result should be an image with transparency.
Some video codecs support the RGBA format and Blender 2.8 let you render with transparency.
One good video codec for this issue is FFmpeg – Quicktime – QT rle / QT Animation.
See in the next image the selection of transparency from Film area.
