Hi – to do that on the default theme, make the code change in the middle section – adapt to your theme
<?php if (have_posts()) : ?>
<?php while (have_posts()) : the_post(); ?>
<div <?php post_class() ?> id="post-<?php the_ID(); ?>">
<h2><a href="<?php the_permalink() ?>" rel="bookmark" title="Permanent Link to <?php the_title_attribute(); ?>"><?php the_title(); ?></a></h2>
<small><?php the_time('F jS, Y') ?> <!-- by <?php the_author() ?> --></small>
<div class="entry">
change this
<?php the_content('Read the rest of this entry »'); ?>
to this
<?php if ($wp_query->current_post == 0) {
the_content('Read the rest of this entry »');
} else {
the_excerpt();
} ?>
</div>
<p class="postmetadata"><?php the_tags('Tags: ', ', ', '<br />'); ?> Posted in <?php the_category(', ') ?> | <?php edit_post_link('Edit', '', ' | '); ?> <?php comments_popup_link('No Comments »', '1 Comment »', '% Comments »'); ?></p>
</div>
<?php endwhile; ?>