Let's say you double posted, or didn't like what you wrote.
Look at the bottom right hand side of the post. There is an 'edit' button there.
Click 'edit'
Now you will see various options.
[save] [go advanced] [delete] [cancel]
If you don't see a delete option, click on 'Go Advanced'.
There should be the above mentioned options. Then click 'delete'.
After clicking 'delete';
Now look again at the left hand side. You should have two options just opened up:
Soft Delete
or
Physically remove post.

Use whichever one you need.
If you double posted 'physically remove post' is the best option.
If you don't want to lose the message of what you wrote but don't want it there for the moment, use the 'soft delete'. The post will be unseeable but can be retrieved by you if you ever want to see it again.