How to customize wordpress sidebar widget

register_sidebar(array(
        'name' => 'Post Sidebar',
        'before_widget' => '<div id="%1$s" class="widget %2$s">',
        'after_widget' => '</div></div>',
        'before_title' => '<div class="titlediv">',
        'after_title' => '</div><div class="widgetdiv">',
    ));

You will have a wrapper div, then your title div and then your widgetdiv.