When you first post, you have to activate the feature that will allow you to delete a post. At the bottom of the "new post" page is this instruction:

To have the option of deleting/editing your post later,
provide a password (CASE SENSITIVE!)

If you don't activate, I believe you are out of luck. If you do activate, there is a time limit on deleting (I believe it is 2 hours, but someone will provide better information)

Administrators always have the power to delete, even if you fail to activate the feature