$block = Mage::app()->getLayout()->createBlock('checkout/cart_shipping');

public function getTest()
{
     $data = 'test';
     return $data;
}

echo $this->getTest();

$myblock = $this->getLayout()->getBlockSingleton('checkout/cart_shipping')->getYourfunctionname();

$myblock = $this->getLayout()->createBlock('checkout/cart_shipping')->getYourfunctionname();