@Bamtan-1306
First off. Have you gotten the invite? (If no, go here, https://archiveofourown.org/invite_requests and request it. It takes 10-14 days to get it normally, but that prevents spammers which is worth the wait.) Then you set up your user name and pass. Once that is done, you go to your dashboard. Click on works, and then you can create one. If it is your own story, not a fan fiction, under the fandom, type/select Original Works, and No Fandom. Anything fetish related I would suggest doing a teen warning for it. Is it M/M, M/F. multi generations, ie adult minor or age gap, and such. When in doubt, click any that could apply.
You don't have to worry about characters or relationship if you don't want to. That is more for fan ficts, like something based on HP could have Harry/Draco, Harry/Ron, Harry/Collin or Harry/Collin/Ron.
Tags are ones it will take a while to get used to. Anything dpr related, I suggest doing, Diapers, Diaper, ABDL, Wetting, messy. If it has teens in it do TBDL, bed-wetting. IF they are horny teens, do that tag. You can type and make up your own ones, but people find stuff by doing tag searches, and by looking at what others have in their bookmarks. If ther is sex, describe what it is. Regression, hypnosis, drugs, fdom, infantilism (Normal and forced), and so on.
Fill in the rest of the stuff, and a description about the story, so people will want to look and read it.
If it is multi chapters, make sure to mark that, and if you know how many, you can enter that, or leave it blank to show its unknown how many, but when its done, mark it as done.
For the actual story. Unless you have done it in HTML code (And even then I still do it this way) Click on Rich text. Then you can copy and paste it from Word, or a text or PDF. When done, click on Preview, then if it looks good POST, if not, go back to edit it more and repeat. Hope this helps. If you need Tag ideas, look at my stories. What's your AO3 username???