To display posts from a single category in your WordPress theme, you should add the following code below the Loop:
<?php query_posts( ‘cat=34’ ); ?>
This code will only showing posts from the category (34) you have selected.
If you want to show posts from more than one category, you can add the IDs separated with commas:
<?php query_posts( ‘cat=33,44,55,66’ ); ?>