The process is very simple to accomplish and it's done with two simple clicks on the divider between the Sidebar and the Finder Window are to the right.

Hide the Sidebar
To hide the Sidebar from view just double click on the divider.  When you do this the Finder will automatically adjust and claim the space that was previously taken by the Sidebar.

Show the Sidebar
By default the Sidebar is visible.  In the event that it has been hidden all you need to do is double click on the extreme left area of the Finder window.  As is the case with hiding, the Finder will automatically adjust and resize to allow the Sidebar to reclaim the space it previously occupied.