global $current_user;
wp_get_current_user();
echo 'Merhaba, ' . $current_user->user_login . "\n";

Kullanıcıya ait gösterebileceğiniz diğer bilgiler:
$current_user->user_login
$current_user->user_email
$current_user->user_level
$current_user->user_firstname
$current_user->user_lastname
$current_user->ID