You go into the thread by using the 'Edit' button and then there should be an option to 'Delete this post'.... you do have a time frame though, which is about 1/2 an hour, however if someone has responded to it in certain ways, then you may not be able to (sorry I cannot explain this one any further
Whichever post you added will have an 'edit' button. If you are within the time limit, edit it and delete text.
You can't delete the whole thread, just what you've written.
I'm useless at these things and have often wondered what to do if I take a look back and seen what I've written is a load of tosh then wondered what to do!!