daivdo's response above is good advice, what you appear to want to do is to make change dynamic, this could be done with an AJAX XHtmlRequest, and a rewrite of the results div done in Javascript called by an onchange event handler associated with the drop down menu.

However to get a feel for the idea, a simple

<!--
<script language="javascript">
function sortResults(){
   var myForm=getElementByID("myForm");
   myForm.submit();
}
</script>
-->
<form action="returnResults.pl" id="myForm" method="POST">
<select name="sortBy">
<option value="byDate">Date...</select>
<input type="text" name="searchCriteria" value="$WHATEVER WE GOT INITI
+ALLY">
</form>
[download]

This will recall the script with a sortBy parameter set, of course it will reload the whole page.

After you get this method to work, have a look at AJAX for inline changes on a page, again as davido said, knowledge has to be acquired and it's not an instantaneous process, however it's a rewarding one.

Happy learning ;)