How to create my own sidebar in Twenty Eleven child theme?

I think I understand what looking for. I’ve put together some code I’ve taken from parts of my themes to give you an example.

For the functions.php:

<?php
add_action( 'after_setup_theme', 'ideatree_init' );
if ( ! function_exists( 'ideatree_init' ) ):
function ideatree_init() {
// REGISTER THE NAV MENUS (3 in this case)
add_action( 'init', 'register_my_menus' );
function register_my_menus() {
register_nav_menus(
    array(
        'main' => 'main' ,
        'secondary' => 'secondary',
        'tertiary' => 'tertiary'
        )
    );
}
// REGISTER THE SIDBARS
if ( !function_exists('ideatree_widgets_init') ) {
function ideatree_widgets_init() {
    register_sidebar( array(
        'name' => __( 'main-widget', 'ideatree'),
        'id' => 'main-widget',
        'description' => 'The Main widget', 'ideatree' ,
        'before_widget' => '<ul><li id="%1$s" class="widget-container %2$s">',
        'after_widget' => '</li></ul>',
        'before_title' => '<h3 class="widgettitle">',
        'after_title' => '</h3>',
    ) );
    register_sidebar( array(
        'name' => 'showcase-widget', 'ideatree',
        'id' => 'showcase-widget',
        'description' => 'The showcase-widget', 'ideatree' ,
        'before_widget' => '<ul><li id="%1$s" class="widget-container %2$s">',
        'after_widget' => '</li></ul>',
        'before_title' => '<h3 class="widgettitle">',
        'after_title' => '</h3>',
    ) );
    register_sidebar( array(
        'name' => 'First Footer Widget', 'ideatree',
        'id' => 'first-footer-widget',
        'description' => 'The first footer widget', 'ideatree' ,
        'before_widget' => '<li id="%1$s" class="widget-container %2$s">',
        'after_widget' => '</li>',
        'before_title' => '<h3 class="widgettitle">',
        'after_title' => '</h3>',
    ) );
    register_sidebar( array(
        'name' => 'Second Footer Widget', 'ideatree',
        'id' => 'second-footer-widget',
        'description' => 'The second footer widget', 'ideatree',
        'before_widget' => '<li id="%1$s" class="widget-container %2$s">',
        'after_widget' => '</li>',
        'before_title' => '<span class="widgettitle">',
        'after_title' => '</span>',
    ) );
    register_sidebar( array(
        'name' => 'Third Footer Widget', 'ideatree',
        'id' => 'third-footer-widget',
        'description' => 'The third footer widget', 'ideatree',
        'before_widget' => '<li id="%1$s" class="widget-container %2$s">',
        'after_widget' => '</li>',
        'before_title' => '<h3 class="widgettitle">',
        'after_title' => '</h3>',
    ) );
}
add_action( 'widgets_init', 'ideatree_widgets_init' );
}
//===============\\ VARIOUS WP THEME SUPPORTS //===================
    if ( ! isset( $content_width ) ) $content_width = 600;
      add_theme_support( 'post-thumbnails' );
      add_image_size( 'bigmutha',580, 180, true ); //Can be used for Portfolio etc...
      add_theme_support('automatic-feed-links');
  }
endif;
?> 

For the sidebar.php:

<div id="sidebar">

  <?php /** Home Page Sidebar */
    if ( is_page('Home') ) { 
    ?>  
      <div id="main-sidebar">
        <?php if ( ! dynamic_sidebar( 'main-widget' ) ) : 
          endif; ?>
      </div>

  <?php /** showcase page sidebar */ }
    if ( is_page('showcase') ) { 
    ?>
    <div id="showcase-sidebar">
        <?php if ( ! dynamic_sidebar( 'showcase-widget' ) ) : 
          endif; ?>
    </div>
  <?php } ?>

</div>

The footer.php file:

<div id="footer">
  <div class="left-footer">
    <?php if ( function_exists ( dynamic_sidebar(3) ) ) : ?>
        <?php dynamic_sidebar (3); ?>
    <?php endif; ?>
  </div>
  <div class="middle-footer">
    <?php if ( function_exists ( dynamic_sidebar(4) ) ) : ?>
        <?php dynamic_sidebar (4); ?>
    <?php endif; ?>
  </div>
  <div class="right-footer">
    <?php if ( function_exists ( dynamic_sidebar(5) ) ) : ?>
        <?php dynamic_sidebar (5); ?>
    <?php endif; ?>
  </div>
</div><!-- End Footer -->
<?php wp_footer(); ?>
</body>
</html>

You should try this out on a testing server first. As I mentioned, this is plucked from two different themes and hasn’t been tested together. It should work for you though. Let me know if you have any questions.