A little help needed!! PHP/Cookies/SQL

<?php
include("config.php");

// connect to the mysql server
$link = mysql_connect($server, $db_user, $db_pass)
or die ("Could not connect to mysql because ".mysql_error());

// select the database
mysql_select_db($database)
or die ("Could not select database because ".mysql_error());

$username = mysql_real_escape_string($_COOKIE['loggedin']);
$query = "SELECT * FROM users WHERE username = \'$username\'";
$rs = mysql_query($query);
$data = mysql_fetch_array($rs);
$firstname = $data['firstname'];
echo "Welcome to Your Account, $firstname";
?>

Can someone please help me get this piece of code right..

This is the error i get atm..
Warning: mysql_fetch_array(): supplied argument is not a valid MySQL result resource in /home/ukgoped/public_html/pedads/account.php on line 41
Welcome to Your Account,

Any help will be appreciated!!

Your incorrect line is here:

$query = "SELECT * FROM users WHERE username = \'$username\'";

Change to:

$query = "SELECT * FROM users WHERE username = '" . $username . "'";

and your good to go.
On a side note, cookies are insecure, you should use sessions instead.

Sessions aren't my strong point..

If you could help me get sessions sorted that would be fantastic! Ijust wouldn't know where to start..

Validating would be a toughy for it, and its been awhile since I've used straight sessions.
All pages must include a session_start() at the top, thats how the sessions are accessed. They are passed using cookies if available, or otherwise with the url:

<?php
session_start();
ob_start();
if (!isset($_SESSION))
{
$_SESSION['username'] = 'Guest';
$_SESSION['loggedin'] = 0;
$_SESSION['ipaddress'] = $_SERVER['REMOTE_ADDR'];
$_SESSION['user_agent'] = $_SERVER['HTTP_USER_AGENT'];
}

if (($_SESSION['user_agent'] != $_SERVER['HTTP_USER_AGENT']) OR ($_SESSION['ipaddress'] != $_SERVER['REMOTE_ADDR']))
{
$_SESSION = array();
if (isset($_COOKIE[session_name()]))
{
set_cookie(session_name(), '', time()-42000, '/');
}
session_regenerate_id();
$_SESSION['username'] = 'Guest';
$_SESSION['ipaddress'] = $_SERVER['REMOTE_ADDR'];
$_SESSION['user_agent'] = $_SERVER['HTTP_USER_AGENT'];
}

Use something of the sorts for a session.php file. Include this into all accessing files:

<?php
include_once('./session.php');
include_once('./config.php');

if (isset($_SESSION['loggedin']))
{
die('Welcome to your account ' . $_SESSION['username']);
}

if (!empty($_POST['submit']))
{
$query = "SELECT `password` FROM users WHERE username = '" . mysql_real_escape_string($_POST['username'])) . "'";
$result = mysql_query($query);
$password = mysql_result($result, 0);
if ($password == $_POST['password'])
{
$_SESSION['username'] = $_POST['username'];
$_SESSION['password'] = $password;
$_SESSION['loggedin'] = true;
header("location: " . $_SERVER['PHP_SELF'] . "?" . SID);
}
else
{
echo 'Username and/or password combination incorrect! Please try again!<br />';
}
}
ob_end_flush();
?>
<form method="post">
Username: <input type="text" name="username" /><br />
Password: <input type="text" name="password" /><br />
<input type="submit" name="submit" value="Submit" />
</form>

Or something of the sorts for a loggin script.

Oh BTW, this is horribly standards uncompliant when it comes to XHTML.