how to call a specific category

Hello! I need to make this code call category id 18 instead of all posts like it currently does. I don’t know much about the code in this particular line and am not sure what needs changing. Any help is appreciated.

<?php $arc_query = new WP_Query('orderby=post_date&order=DESC&showposts=-1'); ?> <?php while ($arc_query->have_posts()) : $arc_query->the_post(); ?> <a href="<?php the_permalink(); ?>" title="<?php the_title(); ?>"><?php echo $post->post_content; ?></a>