Well you can listen for selection changes like this:
<select name="yourselect" onChange="changedOption()">
...
So every time the user changes the selection the changedOption() javascript function will be invoked. How you would show the "related result" is way to open ended to answer, I don't even know where that information should be coming from.
<%@ page contentType="text/html;charset=windows-1252"%>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=windows-1252">
<title>Fruits</title>
<script language="javascript">
function setFruitText(){
document.forms[0].fruitName.value = document.forms[0].fruits.options[document.forms[0].fruits.selectedIndex].text;
}
</script>
</head>
<body>
<form>
<select name="fruits" onchange="setFruitText()">
<option value="1">Apple</option>
<option value="2">Grapes</option>
<option value="3">Mango</option>
<option value="4">Orange</option>
</select>
<input type="text" name="fruitName"/>
</form>
</body>
</html>