in index.php add another line for action
example: action == 'chat'
then load the file that u're using in that file, use template_header() at the beginning of the html and footer() & obExit() at the end
a sample page:
<?phpfunction showPage() { global $username,$txt,$scripturl; global $settings,$yytitle; $yytitle = "NewPage"; template_header(); print <<<EOT<table border="0" width="100%" cellspacing="1" cellpadding="5" class="bordercolor"><tr><td class="windowbg"> ///// Put HTML Codes here \\\\\</td></tr></table>EOT; footer(); obExit();}?>