The CDATA tags need to be included for the page with javascript to validate as XHTML. They also should be commented out for older browser who don't understand CDATA tags:
<script type="text/javascript"> /* <![CDATA[ */ // content of your Javascript goes here /* ]]> */ </script>
To include an external script you can just use this:
<script type="text/javascript" src="hello.js"></script>