Creating a user’s own folder on user registration

You can use the user_register action to hook into the register proces and then create the user directory with wp_mkdir_p.

function create_user_dir($user_id) {
    $user_info = get_userdata( $user_id );

    $upload_dir = wp_upload_dir();
    $user_dir = $upload_dir['basedir'] . '/user_dirs/' . $user_info->user_login;

    wp_mkdir_p($user_dir);
}
add_action( 'user_register', 'create_user_dir');

This example makes a directory in uploads/user_dirs.

http://codex.wordpress.org/Plugin_API/Action_Reference/user_register
http://codex.wordpress.org/Function_Reference/wp_mkdir_p