Now that you have an account here, you should see a link marked "Blog" on your user page.
Click on that and you should see an orange tab that says "Create Blog Post".
Once you click on that, it will prompt you to come up with a title (which can be the title for your fic).and you can either type or copy and paste your text in from there.
If you're on a desktop or laptop, there will be a column off to the right that lets you create a summary (this is the text that will show up as a summary under the updates in 'Wiki Activity'). There is also some tabs that will let you preview what it will look like before actually publishing it, and below that is space that should say "Add Category".
The two that you should add for that are Blog Post and Fan Fiction- I believe including the Fan Fiction tag is enough to have it listed here.
If the content is too explicit for Wiki (not sure what the threshhold is for that) you could alternately set up a pastebin account and leave a link to it in the comments section of Spartan-047's (formerly Camran1005's) constantly updated Fan Fiction Archive, which features links to stories posted on all sorts of external sites like Pastebin, Literotica or Touch Fluffy Tail.