still have this problem

Kontakt.php

class Kontakt extends Strona {
}

class Kontakt_Controller extends Strona_Controller {

    function contact() {
        include 'Form.php';
        $form = new Form();

        return $form->a();
    }

}

code/Form.php

<?php

class Form {

    public function a() { 
        echo 'something';
    }

}

?>

Kontakt.ss

$contact

no errors, site is blank
what is wrong ?

snaip this is probably due to the fact SilverStripe has a built in Form class, you cannot redeclare a class with the same name.

You're getting a white screen probably due to display_errors being turned off in your php.ini file. If you turn that to on, restart apache you should hopefully start to get useful error messages.