Provided your query executes successfully, you can access the columns within the users relation from the properties of the $fetch_users_data object. For example, if you have a column called ‘id’, you could fetch the result like so:
$fetch_users_data = mysql_fetch_object(mysql_query("SELECT * FROM `users` WHERE `username`='".$user_data."'"));
$id = $fetch_users_data->id;
I’d also suggest you to switch over to the MySQLi API for a richer user interface and for peace of mind that the code you’re writing will work in future versions of PHP.