Magento 2 How to select fields in query

Try following way:

$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$resource = $objectManager->get('Magento\Framework\App\ResourceConnection');
$connection = $resource->getConnection();
$select = $connection->select()
    ->from(
        ['ce' => 'customer_entity'],
        ['firstname', 'lastname', 'email']
    );
$data = $connection->fetchAll($select);

Use below code for select particular field from database table.

File path: magento/app/code/Venodr/ModuleName/Model/CustomerData.php

<?php

namespace Venodr\ModuleName\Model;

use Magento\Framework\EntityManager\MetadataPool;
use Magento\Framework\EntityManager\Operation\ExtensionInterface;

class CustomerData
{
    protected $connection;

    public function __construct(
        \Magento\Framework\App\ResourceConnection $resource
    )
    {
        $this->connection = $resource->getConnection();
    }

    public function getQueryData()
    { 
        $query = $this->connection->fetchAll("SELECT firstname, lastname, email FROM customer_entity");
        return $query;
    }
}