To get Twine, go to: https://twinery.org/

Here are some general tutorials that I find useful for beginning in Twine:

(There are two major "story formats"; Harlowe is the default, while SugarCube has better features)

• Twine Cookbook (the official guide which goes a bit into all story formats): https://twinery.org/cookbook/
• Official documentation for Harlowe 3.3.6: https://twine2.neocities.org/
• Official documentation for SugarCube v2: http://www.motoslave.net/sugarcube/2/docs/
• A Quick Twine (2.2+) Tutorial by Allison Parrish (SugarCube): https://catn.decontextualize.com/twine/
• Creating Interactive Fiction: A Guide to Using Twine by Aidan Doyle (Harlowe): https://www.sfwa.org/creating-interactive-fiction-guide-using-twine/
• How to Create Adventure Games using Twine by CCH Learning (a great video tutorial with Harlowe): https://youtu.be/Fp9Sxiv-O-0
• Twine or Treat by GhazWorks (video series with SugarCube): https://youtube.com/playlist?list=PLujRcYYssj76Q_seiXpXnXxzpj1fULa0s
  
• Twine 2 Beginners tutorial series by DigitalExposureTV (video series with Harlowe): https://youtube.com/playlist?list=PLklITFhXtPCCKadv-0Gcbqoj3OCev695D
• Kira's general SugarCube tips: https://guides.writtenhorror.com/kiras/
• The Twine® Grimoire, Vol. 1 & 2 by Grim Baccaris (guide and tips for both Harlowe and SugarCube):
  • https://gcbaccaris.itch.io/grimoire-one
  • https://gcbaccaris.itch.io/grimoire-two

To upload your finished Twine project onto itch.io, follow this guide: https://twinery.org/cookbook/starting/twine2/publishing_on_itchio.html

(Thanks Sjoerd Hekking for suggestions!)