Create a custom log file in Magento 2 to log data .

$writer = new \Zend\Log\Writer\Stream(BP . '/var/log/custom.log');  

$logger = new \Zend\Log\Logger();

$logger->addWriter($writer);

$logger->info("Data".$logmessage);

OR

protected $logger;

public function __construct(\Psr\Log\LoggerInterface $logger)

{  

    $this->logger = $logger;

  }

 $this->logger->info($logmessage);

 $this->logger->debug($logmessage);