[Theme: Twenty Eleven] Summerize posts on homepage

Do you mean excerpts?

If yes, use excerpts instead of content.

The template you’re actually after is “content.php”

You’ll want to change this line:

<?php if ( is_search() ) : // Only display Excerpts for Search ?>
<div class="entry-summary">
    <?php the_excerpt(); ?>
</div><!-- .entry-summary -->
<?php else : ?>
<div class="entry-content">
    <?php the_content( __( 'Continue reading <span class="meta-nav">&rarr;</span>', 'twentyeleven' ) ); ?>
    <?php wp_link_pages( array( 'before' => '<div class="page-link"><span>' . __( 'Pages:', 'twentyeleven' ) . '</span>', 'after' => '</div>' ) ); ?>
</div><!-- .entry-content -->
<?php endif; ?>

To this:

<?php if ( is_search() ) : // Only display Excerpts for Search ?>
<div class="entry-summary">
    <?php the_excerpt(); ?>
</div><!-- .entry-summary -->
<?php else : ?>
<div class="entry-content">
    <?php the_excerpt(); ?>
    <?php wp_link_pages( array( 'before' => '<div class="page-link"><span>' . __( 'Pages:', 'twentyeleven' ) . '</span>', 'after' => '</div>' ) ); ?>
</div><!-- .entry-content -->
<?php endif; ?>

Note the change of the_content()to the_excerpt()

Just changed my blog to this coding and it worked. However, some of the posts don’t have any exerpt at all or a continue reading link. Any ideas how to fix this?

Do not edit the Twenty Eleven theme. It is the default WordPress 3.2 theme and having access to an unedited version of the theme is vital when dealing with a range of site issues.

Create a child theme for your changes. Once your child theme is active, we should be able to help with the customisation.

you can also use More button for the Excerpts creation