Ok, now you have some idea on what the files you got are for so how can you start developing your game?
Try the game first, open the index.html file inside the directory you just unzipped and play the sample game through.
Once you've played it once, open the directory (the one you unzipped) with the editor you chose to start making changes.
script.js file with your editor and find the
script section, as you'll see, all the dialogs you just saw are just a simple list in there. More information can be found in the documentation.
Change one of the dialogs, save the file and reload the game (just like you reload a website).
Play it again and you'll see the dialog changed just like you made it.
Now try adding more dialog to it and you'll quickly get how things are done.
Once you've gotten yourself used to adding dialogs, add a scene as a challenge, that means you'll have to add your image file to the
img/scenes/ directory, more instructions are on the link.
If you manage to do all that, congratulations! You just made your first game and are probably more familiarized with the workflow you'll be using, just make changes, save, reload, try and repeat!