There's a range of frameworks that'll do the trick for you.

An easy and common one is jQuery Dialog (http://jqueryui.com/dialog/)

A small example, given the html:

<div id=dialog-modal title=Basic modal dialog style=display:none>

  <p>Adding the modal overlay screen makes the dialog look more prominent because it dims out the page content.</p>

</div>



<a href=# id=openDialog>Click me</a> 

Assuming you've included jQuery and jQuery dialog on top, add the following javascript:

<script>

$(function() {

   $( #openDialog).on(click, function(){ 

       $( #dialog-modal ).dialog({

          height: 140,

          modal: true

        });

       $( #dialog-modal ).show();

    });

 });

</script>