Since your code is WordPress, I assume you’d append the same code that you have in your meta tag, like this:
<!-- meta tag might look like this -->
<meta name="twitter:description" content="<?php the_excerpt() ?>">
<!-- Tweet button would then look like this, copying the meta content tag from above -->
<ul>
<li><a target="_blank" href="https://twitter.com/share?url=<?php the_permalink() ?>&text=<?php the_excerpt() ?>" class="icon-twitter"></a></li>
</ul>
Watch out to escape the ampersand as & when coding raw HTML.
Find more information in the WordPress support forums.
Does this help you?