You can update your wordpress theme to have such a “Tweet This!” link. How?
Put below, where ever you like in The Loop in single.php.
<a href="http://twitter.com/home?status=I just read <br /><br /> <span style="color:rgb(0,0,0); font-weight: bold;"><br /> <?php</span> the_permalink<br /><br /> <span style="color:rgb(0,153, 0);">(</span><br /><br /> <span style="color:rgb(0,153, 0);">)</span><br /><br /> <span style="color:rgb(51,153, 51);">;</span> <br /><br /> <span style="color:rgb(0,0,0);font-weight:bold;">?></span><br /> <br />" title="Send this page to Twitter!" target="_blank"><br /> Tweet This!</a>
