Does anyone have design experience with blogs? Help please..?

I just finished a few weeks ago my design on the header and sidebar of my home page blog. My question is: I made it where that instead of having 1 column and the sidebar, to 1 column, I fixed them to be together as 1 big column from top to bottom. When I attempt to ad a post the columns go back into 2? Can some one advise me how to correct this issue where I can have 1 column automatically when adding a post or pages…..Or how to fix it?

Thanks in advance..:)
To know and see what I am talking about: Go to