Twine publishes directly to HTML, so you can post your work nearly anywhere. Anything you create with it is completely free to use any way you like, including for commercial purposes.
The latest version of Twine is 2.6.1, released 5 February 2023.
Looking for the 1.x version of Twine? It's on the IF Archive.
The Twine Reference is a guide to the Twine user interface. If you're new to Twine, start here.
Once you're familiar with Twine, you should learn more about the story format you're using. Story formats are like game engines, and determine the features you'll have access to and the way you'll write code.
The Twine Cookbook has advice on how to choose a story format and easy-to-follow examples of how to accomplish common tasks with each of Twine's built-in formats.
The Interactive Fiction Community Forum is a web-based forum for interactive fiction authors.
The Twine Discord is a live chat for Twine authors.
Chris Klimas, who created Twine and leads its development, has a Patreon.
You can also make donations in support of Twine to The Interactive Technology Foundation, a registered 501(c)(3) nonprofit organization that provides support for the infrastructure of the interactive fiction community.
Some story format developers also accept donations. Visit the web site of the story format you use to find out how you can support continued development.
Non-financial support is important, too. You can help someone out with a question, contribute to the Cookbook, create tutorials of your own, help fix bugs in Twine or its story formats, or translate Twine's user interface to another language.