How do I use wp_query for WordPress search?

You need to combine your search query with a new WP Query…

That would be something like this, on your search.php

<?php

global $wp_query; // get the global object

$thesearch = get_search_query(); // get the string searched

// merge them with one or several meta_queries to meet your demand
$args = array_merge( $wp_query->query, array( 
   'meta_query' => array(
    array(
        'key' => 'field_to_seach',
        'value' => $thesearch,
        'compare' => 'IN'
    )
)
    ));
query_posts( $args ); // alter the main query to include your custom parameters

?>