header('location:...'.php') before html can't make my code work

Question

I´ve read many of the questions and answers here regarding header('location.') and tried th adjust my code below according to it, but I can't seem to make it work for my login page. I get the common error

Warning: Cannot modify header information - headers already sent by (output started at /Applications/MAMP/htdocs/Curie/webroot/nobel_login.php:1) in /Applications/MAMP/htdocs/Curie/src/CUser/CUser.php on line 33

I also know that header() must be called before any actual output is sent, either by normal HTML tags, blank lines in a file, or from PHP. How shall I write this code (pls see below) to make it work? Thank you in advance for input and advice.

   <?php
/**
 * A class for handeling login
 *
 */
class CUser extends CDatabase {
    /**
     * Members
     *
     */
    private $database;
    /**
     * Initiate child and parent constructor
     *
     */
    public function __construct($database) {
        parent::__construct($database);
    }
    /**
     * Check if user and password is okey and reload page
     *
     */
    private function Login() {
        if(isset($_POST['login'])) {
          $sql = "SELECT acronym, name FROM USER WHERE acronym = ? AND password = md5(concat(?, salt))";
          $params = array(strip_tags($_POST['acronym']), strip_tags($_POST['password']));
          $res = $this->ExecuteSelectQueryAndFetchAll($sql, $params);

          if(isset($res[0])) {
            $_SESSION['user'] = $res[0];
          }
            // Den ska skrivas ut innan html skrivs ut. 
      header('Location: nobel_login.php');              
        }

        // Logga ut användare / Logout the user
        if(isset($_POST['logout'])) {
        unset($_SESSION['user']);
        header('Location: nobel_login.php');
        }
    }



    /**
     * Verify if user is logged in or not and return message
     *
     */
    private function IsAuthenticated($acronym) {

        if($acronym) {
            $acronym = $_SESSION['user']->acronym;
            $name      = $_SESSION['user']->name;
          $output  = "Du är inloggad som: {$acronym} ({$name})";
        }
        else {
          $output  = "Du är för närvarande inte inloggad.";
        }

            return $output;
    }
    /**
    * Create login form
    *
    */


    private function LoginForm($acronym) {
    $form = "<form method='post'><fieldset><legend><br></legend>";
    $form .= "<p><em>Du kan logga in med vinga:vinga eller pippi:pippi</em></p>";
    $form .= "<p><label>Användare:</label><br><input type='text' name='acronym' value='pippi'></p>";
    $form .= "<p><label>Lösenord:</label><br><input type='password' name='password' value='pippi'></p>";
    $form .= "<p><input type='submit' name='login' value='Login'> ";
    $form .= "<input type='submit' name='logout' value='Logout'></p>";
    $form .= "<p><b>{$this->IsAuthenticated($acronym)}</b></p>";
      $form .= "</fieldset></form>";
          return $form;
    }

    /**
     * Sum it all up and create HTML output
     *
     */
    public function UserLogin() {
        // Get parameters
        $acronym = isset($_SESSION['user']) ? true : false;
        // Create HTML output based on parameters
        $html = $this->Login();
        $html .= $this->LoginForm($acronym);
            return $html;
    }
}