There already is a header, it’s just that the theme was made to assign what your are referring to as the “left sidebar” as the actual header in the code. There are a few options for you , but the simplest I can think of is maybe edit the HTML of the ‘header.php’ file to include a div element (avoid a class or ID of “header”; it’s taken) with what you need in it and style with CSS in your main stylesheet, etc.
If it’s going to have a menu or nav, you’ll need to add the appropriate editable regions for a menu to be able to add/edit menu items from the back end.
