Use a shortcode to display custom meta box contents

yeah, the question is a bit unclear. but i think you need shortcode attributes

function get_custom_field_fn( $atts) {    
global $post; 

 extract( shortcode_atts( array(
        'meta' => ''
    ), $atts ) );


 $custom_field_value = get_post_meta($post->ID, $meta, true); 
 return $custom_field_value; 
} 

add_shortcode('cf_name', 'get_custom_field_fn');

see the codex: http://codex.wordpress.org/Shortcode_API#Attributes