Duplicate - sorry!

Under your post you want to delete, click on "Flag". In the pop-up that appears, click "Report". In that pop-up, scroll down and click "This is my post and I want to delete it" then scroll all the way to the bottom of that same pop-up and click the "Send Report" button.

That will not delete your post immediately, but will send a message to the moderation queue asking moderators to delete it. Deletion usually happens reasonably quickly, but how fast depends on how many volunteer moderators are online.