Hello
There may be a way to do exactly what you want using the more tag and stuff though I am unsure, but this might also accomplish generally what you want.
<?php if ( have_posts() ) : while ( have_posts() ) : the_post(); $postcount++; ?>
<h2 id="post-<?php the_ID(); ?>"><a href="<?php the_permalink() ?>" rel="bookmark" title="<?php the_title(); ?>"><?php the_title(); ?></a></h2>
<p class="date"><?php the_time('F jS, Y') ?> <!-- by <?php the_author() ?> --></p>
<?php if ($postcount == 1) { ?>
<?php the_content("Continue reading: " . the_title('', ' »', false)); ?>
<?php } else { ?>
<?php the_excerpt() ?>
<p><a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>">Read more of <?php the_title(); ?></a></p>
<? } ?>
<p class="postmetadata">Posted in: <?php the_category(', ') ?> | <?php comments_popup_link('No Comments ?', '1 Comment ?', '% Comments ?'); ?> <?php edit_post_link('Edit',' | ',' '); ?></p>
<!--
<?php trackback_rdf(); ?>
-->
<?php endwhile; ?>
This will display the most recent post in it’s entirety, than the older posts will just have the excerpt.
The excerpt by default will use the first 175 letters of the post (or something like that) or if you enter text into the optional excerpt field of a post (in the dashboard) that will be used instead.
So, it is not exactly what you we’re asking for but does almost the same thing.
This would go in index or home.php depending on which file you are using.
Hope it is helpful. ??