Add more widget areas to a theme

You have to add this code in the functions.php file:

/**
 * Register new sidebar
 *
 */
function new_sidebar_widget_init() {

register_sidebar( array(
    'name' => 'new-sidebar',
    'id' => 'new-sidebar',
    'before_widget' => '<div id="new-sidebar">',
    'after_widget' => '</div>',
    'before_title' => '',
    'after_title' => '',
) );
}
add_action( 'widgets_init', 'new_sidebar_widget_init' );

and then call the sidebar from a template to display it:

<?php  dynamic_sidebar( 'new-sidebar' ); ?>