Meta tag with featured image's url?

I’m trying to put a meta tag in my header that will pull the featured image’s url from a post.

Why can’t I get this to work? Here’s my code:

<meta name="my_meta" content="<?php wp_get_attachment_url();?>"/>

I’ve tried all kinds of things but I’m still pretty noobish here. For instance,

<meta name="twitter:image" content="<?php get_permalink();?>"/>

works just fine as far as the formatting and syntax goes. How can I stick the featured image’s URL in there instead?

Thanks!