You can delete you post immediately after you post it, but on some board like service board which I noticed, you will have to wait for 24 hours before you can delete your post there. But if it is a topic, you can not delete it.
If the thread you created has posts from other users, then you can't delete that thread. Only the moderators can. If you had that privilege, it would basically mean that you are deleting other user's posts together with yours. But if no one else posted in your thread (only you did), you can delete it. I don't know if this isn't possible in certain boards like Services.