For this, you need to edit the file ‘meta-boxes.php’ located in wp-admin/includes/ folder.
Open that file in NotePad and find the following code:
$visibility = ‘public’;
$visibility_trans = __(‘Public’);
Now, all you need to do is change it do the following:
$visibility = ‘private’;
$visibility_trans = __(‘Private’);
Done.
via WordPress › Support » How to set new posts visibility to private by default ?.