Welcome Sir Knight,
Making any kind of game on Tumblr is pretty much impossible since that's just a social media platform, unless you meant Twine which would make more sense. AO3 is more for non interactive stuff since all they really have is a basic text editor.
If you want to get started I'd recommend the good old RTFM. You can find multiple articles in tutorials for anything site and writing related here:
https://chooseyourstory.com/help/. Starting out with a smaller story using the basic editor is a good idea when you don't have much experience writing interactive stuff.
Either way good luck with your project.