Drupal 8 Basic form example

MyForm.php

<?php
namespace Drupal\my_module\Form;

use Drupal\Core\Form\FormBase;
use Drupal\Core\Form\FormStateInterface;

/**
 * Class TestAjaxFormSelect.
 */
class MyForm extends FormBase {
  
  /**
   * {@inheritdoc}
   */
  public function getFormId() {
    return 'my_unique_form_id';
  }
  
  /**
   * {@inheritdoc}
   */
  public function buildForm(array $form, FormStateInterface $form_state) {
    // Get the form values.
    $values = $form_state->getValues();
    
    // Element(s).
    $form['my_select'] = [
      '#type' => 'select',
      '#empty_value' => '',
      '#empty_option' => '- Select a value -',
      '#default_value' => (isset($values['my_select']) ? $values['my_select'] : ''),
      '#options' => [
        1 => 'One',
        2 => 'Two',
        3 => 'Three'
      ],
    ];

    $form['submit'] = [
      '#type' => 'submit',
      '#value' => 'Submit my form',
    ];
    
    return $form;
  }
  
  /**
   * {@inheritdoc}
   */
  public function submitForm(array &$form, FormStateInterface $form_state) {
    // Do something with $form_state->getValues();
  }
  
}