Add this to your head tag:
It you are using Twitter Bootstrap, add this class to you main/wrapper div:
<div id="Wrapper" class="container">
Otherwise look up some info on css media queries and add them to your style sheet.
EDIT:
Since you are using WP locate your functions.php and add the files;
function my_scripts_enqueue() {
wp_register_script( 'bootstrap-js', get_template_directory_uri() . '/js/bootstrap.min.js', array('jquery'), NULL, true );
wp_register_style( 'bootstrap-css', get_template_directory_uri() . '/css/bootstrap.min.css', false, NULL, 'all' );
wp_enqueue_script( 'bootstrap-js' );
wp_enqueue_style( 'bootstrap-css' );
}
add_action( 'wp_enqueue_scripts', 'my_scripts_enqueue' );
As for media queries, have a look at this post;
http://stackoverflow.com/questions/12045893/which-are-the-most-important-media-queries-to-use-in-creating-mobile-responsive
hope that helps