The requirements are the following:
PhpMyAdmin(MySQL)
And FTP server such as FileZilla
And obviously a website.

Now to get started. You must first enter this MySQL query:

CREATE TABLE `user_online` (
`session` char(100) NOT NULL default '',
`time` int(11) NOT NULL default '0'
) TYPE=MyISAM;

Once thats put in, the new table will be called user_online. Now that we have the table in our database we have to transfer that to your website.

You can call this user_online.php, or whatever you want to call it. Once you make that new file you must put this into that page:

<?
session_start();
$session=session_id();
$time=time();
$time_check=$time-600; //SET TIME 10 Minute

$host="localhost"; // Host name
$username=""; // Mysql username
$password=""; // Mysql password
$db_name="test"; // Database name
$tbl_name="user_online"; // Table name

// Connect to server and select databse
mysql_connect("$host", "$username", "$password"or die("cannot connect to server";
mysql_select_db("$db_name"or die("cannot select DB";

$sql="SELECT * FROM $tbl_name WHERE session='$session'";
$result=mysql_query($sql);

$count=mysql_num_rows($result);

if($count=="0"{
$sql1="INSERT INTO $tbl_name(session, time)VALUES('$session', '$time'";
$result1=mysql_query($sql1);
}
else {
"$sql2=UPDATE $tbl_name SET time='$time' WHERE session = '$session'";
$result2=mysql_query($sql2);
}

$sql3="SELECT * FROM $tbl_name";
$result3=mysql_query($sql3);

$count_user_online=mysql_num_rows($result3);

echo "User online : $count_user_online ";

// if over 10 minute, delete session
$sql4="DELETE FROM $tbl_name WHERE time<$time_check";
$result4=mysql_query($sql4);

mysql_close();

// Open multiple browser page for result
?>

You have to put the name of your database that you put the user_online table in, your password to your database, your database name, and what host you are using. Like vistapanel.net's is sql210.vistapanel.net. Thanks for reading this tutorial, hopefully this helped you.