more on General guidance in JS programming

I recently posted under the title:  General guidance in JS programming".  Someone replied but I do not see how to reply to that comment.  So I'm starting another post.

I recent posted a question to which An0n3m0us replied, in part, as follows:

"Start here:

https://www.khanacademy.org/computing/computing-programming/programming

The videos/articles will show you almost everything that you need to know to be able to code in Processing JS.

Once written the code, it will automatically output to the canvas and show you what you have created with it."

Well, I have viewed the above link several times and I'm still confused.  I do not see where it explains what program to use to write this code.  Is this done at the Command prompts?  Or is there a special program I need to download?  or just what?

And you say "it will automatically output to the canvas" - I've no idea what you mean.  Since I can't determine where to start writing my code, how is "it" going to do anything automatically?  And what do you mean by "canvas"?

It seems I am missing something here and I'm hoping someone can steer me in the right direction. Thanks