Many people are so concerned about the gear they think they need, that they don't put their energy where it needs to be--tell an interesting story. If you are shooting for YouTube and the like, then just have a vague idea of what you have to say and wing it.

In contrast, the pros begin with "pre-production" and that is all about the script. By the way, "production" is the shooting and recording phase and "post-production" is the editing phase. We're just talking about pre-production right now.

Here are the basics in telling a story. The emphasis here is in the realm of instruction, promotion, and information. Hollywood uses a slightly different appoach with an emphasis on emotional connections and development. Showing someone how to treat a tree for crown gall is not quite the same, but that's more of what we do.

Start with an outline of what you want to say. Don't worry about what anything will look like yet. You can divide your story in acts or simply parts. 
Part 1 is the introduction. This can include something attention-getting or simply tell the audience what the purpose of the video is and who should watch it.
Part 2 is the meat of your story.  All the points you want to make are presented in detail here.
Part 3 is the conclusion where you remind the audience the purpose and suggest how this information can or will affect them.

I use this reminder when I outline my scripts: "Tell'em what you want to tell them, tell them, tell'em what you told them."  

Once you know what the story is and what the details are, style becomes a consideration. How will you tell your story? Your choices include:
a. Unseen narrator.
b. Onscreen host who will also narrate.
c. Host or narrator with interviews included.
d. Interviews only. (Very difficult to do, but great when one inteview flows into another and another to tell the whole story.) These are often done with "word cards" (onscreen titles) to tie things together.
e. No narration, just music and natural sound that support the visuals.
f. Combinations of segments using variations of above.

Some will work better than others, depending on your audience and the time you can hold their attention.

After you've come up with a style, apply that to your outline to develop a script. The emphasis should be on the audio--narrator, interviews, etc. Some people start to fret about the video shots at this point. You are scripting---anything is possible right now.  Write the audio part first and then work on the video.

Below I have provided your a link for a television script format file you can download. I've been using it for years with MS Word. It keeps the video and audio in separate columns. It uses tables to keep each shot aligned with its audio. It is for educational, informational-type programs, not teleplays or feature films--that's a different format and is greatly modified once you get into editing the film. This television script format I use can carry you through all phases of the process.

Whenever you write for narration or radio, be sure to read it aloud. We tend to speak in shorter sentences than we write and you'll find some of your writting is too long-winded to work well as audio. Test it out.  Does it make sense? Does it sound like something someone would say? Remember, your audience can't see the punctuation so it is all about how it sounds. 

Pre-production is usually the longest part in a production's timeline. It includes the all important script, review and revisions, and planning for interviews, shooting, graphics or other illustrations, etc.

Step 2: The Shoot, will be next.