$customer = Mage::getSingleton(‘customer/session’)->getCustomer();

$email = $customer->getEmail(); // To get Email Address of a customer.
$firstname = $customer->getFirstname(); // To get Firstname of a customer.
$lastname= $customer->getLastname(); // To get Lastname of a customer.
$id =$customer->getId(); // To get Customer Id

//Get Details Using Customer Id
$customer = Mage::getModel(‘customer/customer’)->load($id)->getData();