here you go,
The following returns all posts that belong to category 1 and are tagged “apples”
query_posts( 'cat=1&tag=apples' );
Add this inside the page before the loop
change ‘1’ and ‘apples’ to whatever category and tag you want.
If you must use query_posts(), make sure you call wp_reset_query() after you’re done.
look here for more info,