How to place sidebar on a top-side ?

Hi,

After seeing this post : can widgets be placed on the side? ,
I tried to put the sidebar on one side of the page instead of the bottom-center, but I cannot find how to tell it the exact location where it should be displayed, in the .php file code and css (for example : on the left-top of such a frame, or/an within which frame…)

Can you specify more exactly in which .php file we should insert the sidebar, and in which part of the template (which <div>, for example) ?

Thanks a lot for your precious help !