Theme's general layout options?

Theme: Twenty Ten.
Who is in the right perception and could suggest me please on how to “kill” second column under the header?
I did it at first somehow (before “upgrade”) and I was scared because for some reason it was only one place to log in to the website even for “admin”. Somehow I’ve got the way to put “log in” URL into the “menu” – line of URLs but for now I can not find again how to delete second column. I never thought that making a URL to a webpage could be so difficult to do. Why do I need it:
For some reason entire layout of the website is under the same configuration. It would be really good to keep second column for 1st page (titular one) but at the pages of the “Forum” such second column narrows entire forum’s text width down to 1/3rd of the header’s banner width leaving most of the computer’s screen empty.
The link: http://dev4ata.com/?forum=forum