Websites such as Archive of Our Own (AO3) are good options. They offer a free space for sharing your stories and have a wide user base. But make sure to follow their guidelines.
You could try social media platforms like Facebook or Instagram. But keep in mind, the format might be a bit limiting. Or, you could submit to literary magazines or websites that accept submissions. Do some research to find the ones that fit your story's genre and style.
Yes, Wattpad is a free platform where you can post your original story. It has a large user base of readers who are interested in different genres of stories. Another free option is Medium. You can create an account and start sharing your story right away. DeviantArt is also free and can be a good place if your story has some artistic elements that you can pair with relevant artworks.