Magento 1.x:
Mage::getStoreConfig('carriers/shipper/active')
Magento 2.x (using constructor injection):
protected $scopeConfig;
public function __construct(\Magento\Framework\App\Config\ScopeConfigInterface $scopeConfig) {
$this->scopeConfig = $scopeConfig;
}
Then you can do
$this->scopeConfig->getValue('path/to/config', \Magento\Store\Model\ScopeInterface::SCOPE_STORE);