There isn’t anything out of the ordinary that you need to do to create a dialog window in EE. The only EE related code you’d need is outputting the content with a simple exp:channel tag.
That being said—my go to dialog window script is Fancybox (http://fancyapps.com/fancybox/) which is very well made and sports many desirable features associated with dialog windows.
With this solution, you could accomplish what you want like this:
(Inside script tags)
$(document).ready(function() {
$(".fancybox").fancybox();
});
And then this
<a href="#contentclass=fancybox">Link to content</a>
<div id="content">
{exp:channel:entries channel="blog" entry_id="1234" dynamic="off"}
{body}
{/exp:channel:entries}
</div>