If you want something else than the default relevancy ranking, you can use
order parameters for the search query. Orderby accepts
$post variable attributes and order can be asc (ascending) or desc (descending). The most relevant attributes here are most likely post_date and comment_count.
If you want to give your users the ability to sort search results by date, add something like this to your search results page:
<?php echo '<p><a href="' . get_bloginfo('url') . '?s=' . get_search_query() . '&orderby=post_date&order=desc">Order results by date<a></p>'; ?>
Order by relevance is either
orderby=relevance or no orderby parameter at all.