You have to wait 30 minutes after someone on the topic you want close posts. Then go to edit on the the first post (if you made the topic) the press delete. The topic will then be closed.
Okay, first of all: You double post a lot; STOP IT.
Second of all, on the right bottom side of your begining post, you will see a button that says 'Edit' at the very end. Click it. Then, once you're taken to the edit screen, look under your avatar area; you'll see two buttons: 'Delete' and 'Close'. Click the one you want.