One first installation you must have set admin user. You can login from ursite/admin.php (ie.www.yes.com/admin.php)
What version are you using ?
if u are using version 5x and forgot about admin password and login name try this code. ONce u run this file from root directory of ur nuke system, log in as username(whatever u have changed "$urname" to and password is "test". ONce u login go to Administration menu and change admin password.
<?php
include("mainfile.php");
echo "<h1> Updating Admin to the nuke System </h1>";
global $dbi;
$urname="xxx";// change this xxx username
$result=mysql_query("update nuke_authors set aid='$urname',pwd ='098f6bcd4621d373cade4e832627b4f' where name='God'", $dbi);
if( ! $result)
die ("Sorry I Could not update Admin account : ".mysql_error());
Echo " DOne ! NOw delete this file ";
?>