Whatever kind of video you want to make, thorough planning is vital. Making a video can be a time consuming and complex process. Without proper planning you cannot hope to convey a story or message effectively.
Don't over complicate the basic idea of your video. You should make sure your idea is very simple and clear or your audience will lose interest.
It might sound silly, but you should be able to convey the idea of your video in one sentence. For example: Aliens invade Earth to wipe out humans but just when things look hopeless, they all catch a cold and die.
That's it, that's your story. All else is just detail to support this or to add interest. Try doing this with your idea.
The screenplay details all the scenes in your video and includes any dialogue that you want your actors to say. It should describe your shots and sequences, where they are set, do they need any special effects, what other sounds or music might be needed.
A storyboard is for planning the shots you need. You don't have to be able to draw very well - stick men will do fine. Creating a storyboard makes you think about the visual style of the video and the types of shots you need. You can use a storyboard instead of a screenplay if the video is fairly short and simple.