Query to show one post on home page

I cannot seem to figure out where to place the query to show only one post in the home.php file of my template. It uses this file for the home page. Here is the code:

<?php if (have_posts()) : ?>
<?php while (have_posts()) : the_post(); ?>
<div class="fullbox" id="post-<?php the_ID(); ?>">
<h3><?php the_category(', ') ?></h3>
<div class="fullbox_content">
<h1><a href="<?php the_permalink(); ?>" rel="bookmark" title="<?php the_title(); ?>"><?php the_title(); ?></a></h1>

Any help would be appreciated!