APEX

I would like to have a small Message Window Pop-Up whenever the user selects a particular item from a Drop Down List.

I have minimal experience with Ajax and struggle greatly every time I need to use it.
But I suspect I'll need to create a Hidden Temporary Item and Use Ajax.
Beyond that I really dont have an idea.

I also know that I dont want to call a Page. I just want a small message window that contains some help text and that the user can quickly close.

For another purpose I have used the following:

<script language="JavaScript" type="text/javascript">
function waps_javaprompt () {
var Hours = prompt("How Many Hours?");
var Minutes = prompt("How Many Minutes?");
$x('P142_WAPS_HOURS').value = Hours;
$x('P142_WAPS_MINUTES').value = Minutes;
}
</script>

I dont understand it that well, but it met my needs for its application, and it seemed pretty simplistic.

I was hoping to be able to modify the above code to meet my current purposes,
But there are many differences:

1. I need the popUp message to be conditional.
2. The condition is based upon a particular selection from a Drop Down list
3. I dont want prompts in the Window. Only a sentence or two of text for the help message.

Can anyone help me?