How to insert data in the database in Symfony 3 issue

Usually, the favorite way to deal with such cases is to create a new entity (stored in AppBundle/Entity directory - which as I can understand, you already created it), and based on that entity, you need to create a new form (stored in AppBundle/Form directory).

Now, the problem with forms is that you can create them in several ways. One way is the one I've already told you; another way is to create it in the controller method, as you did.

To summarize, the following is just an example, using the first way, and using symfony >= 2.8:

//1. Create an Entity class, using a symfony console command (after completing all steps, you'll end with the directory AppBundle/Entity and inside it a new php file Contact.php):
$ php app/console doctrine:generate:entity //and follow the interactive steps, and let's say you need the following columns: name (varchar:255), email (varchar:255), website (varchar:255), and comment (varchar:255).

//2. Create a new Form, based on that Entity class (after completing all steps, you'll end with the directory AppBundle/Form and inside it a new php file ContactType.php):
$ php app/console doctrine:generate:form AppBundle:Contact

//3. In the controller method:
use AppBundle\Entity\Contact;
use AppBundle\Form\ContactType;
//...
/**
 * @Route("/contact/", name="contact")
 * @Method({"GET","POST"})
 */
public function contactAction(Request $request){
    $contact = new Contact();

    //for symfony >= 2.8
    $form = $this->createForm(ContactType::class, $contact, [

    //or if you're using symfony < 2.8, replace the above line with this:
    $form = $this->createForm(ContactType, $contact, [
        'action'=>$this->generateUrl('contact'),
        'method'=>'POST'
    ]);
    $form->handleRequest($request);
    if($form->isSubmitted() && $form->isValid()){
        //...more stuff pre-insertion here if needed
        $em = $this->getDoctrine()->getManager();
        $em->persist($contact);//persist the contact object
        $em->flush();//save it to the db
        //...more stuff post-insertion here if needed
        return $this->redirectToRoute('homepage');
    }
    return $this->render('contact/content.html.twig', array(
        'form' => $form->createView(),
    ));
}

//4. In contact/contact.html.twig:
{{ form_start(form) }}
    <div class="text-fields">
        <div class="float-input">
            {{ form_row(form.name,{ attr:{ name:'name',id:'name',placeholder:'Name' } }) }}
            <span><i class="fa fa-user"></i></span>
        </div>
        <div class="float-input">
            {{ form_row(form.email,{ attr:{ name:'email',id:'email',placeholder:'e-mail' } }) }}
            <span><i class="fa fa-envelope-o"></i></span>
        </div>
        <div class="float-input">
            {{ form_row(form.website,{ attr:{ name:'website',id:'website',placeholder:'website' } }) }}
            <span><i class="fa fa-link"></i></span>
        </div>
    </div>
    <div class="comment-area">
        {{ form_row(form.comment,{ attr:{ name:'comment',id:'comment',placeholder:'Message' } }) }}
    </div>
    <div class="submit-area">
        <input type="submit" value="Send"/>
    </div>
    <div id="msg" class="message"></div>
{{ form_end(form) }}

But please pay attention, as if you are using a symfony version < 2.8, then you should look here to see how to render text types (you'll need texttype, emailtype, and textareatype --- OR you can let them as they were generated, as is enough), but if you're using symfony >= 2.8, then all you need is to import, at the top of your ContactType class, the respective classes for each type you're using:

use Symfony\Component\Form\Extension\Core\Type\TextType;
use Symfony\Component\Form\Extension\Core\Type\TextareaType;
use Symfony\Component\Form\Extension\Core\Type\EmailType;

And, when building the form:

//...
$builder
    ->add('name',TextType::class)
    ->add('email',EmailType::class)
    ->add('website',TextType::class)
    ->add('comment',TextareaType::class)
;