Thanks Randy, I will add a pointer to
Posting Issues, Formatting and Topic Types, and other Forum FAQs to the mother of all HB FAQs, the
FAQs and Favorites Digest. It's the stock phpBB FAQ, which documents the bare minimum forum usage, including
How do I edit or delete a post?
Unless you are the board admin or forum moderator you can only edit or delete your own posts. You can edit a post (sometimes for only a limited time after it was made) by clicking the edit button for the relevant post... Please note that normal users cannot delete a post once someone has replied.
It's the last sentence that applies to your case. If nobody has posted a reply after your post, you're free to delete it. This comes in handy, for example, if you accidentally double post. Once someone has posted a reply, all posts prior to it cannot be deleted except by moderators, so the delete button

is not shown. This is to avoid disrupting the flow of the thread if person X posts, person Y replies, then person X decides to delete their original post, removing the context of person Y's reply.
Sometimes it happens that someone posts immediately after you accidentally double post, preventing you from tidying up after yourself. In that case just edit the text to "DOUBLE POST - PLEASE DELETE" or similar text and one of the moderators will tidy up.