Hey there! So if you're wanting to post something on YWS, you go to where it says 'Publishing Center' on the black bar up the top of the page. You click on that, then on the next page there's a red button that says 'add new work'. Click on that and you'll be able to post!
Like TriSARAHtops said, you can publish a story/poem at the publishing center. If you want to write a chapter book you could make a folder at your portfolio specially for that story and publish one chapter at a time.
Spoiler
If you're reading this I hope something good happens to you today <(0^0)>
If plan A doesn't work, stay calm, the alphabet has 25 more letters to go.
