Okay so I took some screenshots and hopefully that will help.
From the main page, you can select “Start a Discussion”

It’ll pop up a little window where you can enter your story into the body section, enter a title, and add tags.

Then, you pick a primary tag. This is where your post will go. For this example, we’ll use Fiction since you’re asking about posting a story.

Then, you are able to select additional tags, based on your story’s content.

Then, you can post your story!
To make a multi-part story, just add your additional chapters/parts as replies to the original story thread. To do this, you’ll open your story thread back up, then you’ll see the following buttons:

The reply button will let you add a reply just as before, and, if you click the dropdown arrow, it will let you edit tags.
The follow button will allow you to follow a post, if you want to be notified of new replies (this is especially useful if you want to be sure you catch new posts on a story you like, by someone else… or you can follow your own stories to be notified if anyone comments. (this is also where you can choose to ignore a post if you need to and you won’t see it come up in your feed))
Please feel free to ask any additional questions, but I think I covered your questions thus far 🙂