Plugin and option page

Hello everyone,
I’m here because I’ve a problem about the option page for my plugin. I just want to draw 7 checkboxes, and to save their values in the option array shareit-options. I tried many different things but nothing works. I give you a piece of the code I tried :

add_action('admin_init', 'shareit_admin_init');
function shareit_admin_init(){
register_setting( 'shareit_options', 'shareit_options', 'shareit_options_validate' );
add_settings_section('shareit_main', 'Boutons de partage à afficher', 'shareit_section_text', __FILE__);
add_settings_field('chkbx1', 'Twitter', 'shareit_setting1', __FILE__, 'shareit_main');
}

function shareit_section_text() {
echo '';
}

function shareit_setting1() {
$options = get_option('shareit_options');
if($options['chkbx1']) { $checked = ' checked="checked" '; }
echo "<input ".$checked." id='chkbx1' name='chkbx1' type='checkbox' />";
}

function shareit_options_validate($input) {
return $input;
}

register_activation_hook(__FILE__, 'add_defaults_fn');

function add_defaults_fn() {
$arr = array("twitter" => "on");
update_option('shareit_options', $arr);
}

Thanks for help!