Then just
<td width="28%" valign="middle" align="left" class="smalltext">
(Your own text here "xStyle by yournamehere) <a href="Your www site (url)" target="_blank">(your name or powered by)</a>
</td>
You can find it in index.template.php
Full code of copyright is here
// Show the "Powered by" and "Valid" logos, as well as the copyright. Remember, the copyright must be somewhere!
echo '
<div id="footerarea" style="text-align: center; padding-bottom: 1ex;', $context['browser']['needs_size_fix'] && !$context['browser']['is_ie6'] ? ' width: 100%;' : '', '">
<table cellspacing="0" cellpadding="3" border="0" align="center" width="100%">
<tr>
<td width="28%" valign="middle" align="left" class="smalltext">
Theme edited by <a href="www.lsclan.net" target="_blank">unleshead</a>
</td>
<td valign="middle" align="center" style="white-space: nowrap;" class="smalltext">
', theme_copyright(), '
</td>
<td width="28%" valign="middle" align="right" class="smalltext">
<a href="http://validator.w3.org/check/referer" target="_blank" style="font-size:10px;">XHTML</a> | <a href="http://jigsaw.w3.org/css-validator/check/referer" target="_blank" style="font-size:10px;">CSS</a>
</td>
</tr>
</table>
</div>
<br />';