How to store data in database with Form API?

Question

I'm new to developing Drupal 7 modules, and I can't figure out why my custom module's form data is not stored in my database's table called 'media_skins'.

I followed the info explained in Creating modules - a tutorial: Drupal 7.x but I can't get it working. What's wrong with my code?

Here's my code:

function media_skins_menu() {
  $items = array();
  $items['admin/config/content/media_skins'] = array(
    'title' => 'Media Skins',
    'description' => 'Configuration for Skins module',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('media_skins_form'),
    'access arguments' => array('access administration pages'),
    'type' => MENU_NORMAL_ITEM,
  );

  return $items;
}

function media_skins_form($form, &$form_state) {

  $form['media_skins_test'] = array(
    '#type' => 'textfield',
    '#title' => t('Test field'),
    '#size' => 20,
    '#maxlength' => 20,
    '#description' => t('Test info for the field.'),
    '#required' => FALSE,
  );
  return system_settings_form($form);
}

function media_skins_form_validate($form, &$form_state) {
    // For now, I will no validate anything
}

function media_skins_form_submit($form, &$form_state) {
  $form = db_insert('media_skins')->fields(
    array(
      'start_date' => $form_state['values']['media_skins_test'],      
    )
  )
  ->execute();
  drupal_set_message('Data stored in database successfully'); 
}

Any help will be very appreciated.

EDITING THE SCENARIO

After reading your comments, I tried the following, but I get "Unexpected error, try it again":

function media_skins_form($form, &$form_state) {
  $form['media_skins_test'] = array(
    '#type' => 'textfield',
    '#title' => t('Test field'),
    '#size' => 20,
    '#maxlength' => 20,
    '#description' => t('Test info for the field.'),
    '#required' => FALSE,
  );
  $form['submit'] = array(
    '#type' => 'submit',
    '#value' => t('Enviar'),
  );
  return $form;
}

Answer 1

To use your own form submit handler, you should not use system_settings_form in your form, instead just return your form.

If you are just going to save your value in your submit handler, go with system_settings_form. To get the values, use variable_get('your form field name'). In your case, you can use variable_get('media_skins_test');

Answer 2

You have to variable_get like below.Now you can get the values why because it will store in the variable table.

function media_skins_menu() {
  $items = array();
  $items['admin/config/content/media_skins'] = array(
    'title' => 'Media Skins',
    'description' => 'Configuration for Skins module',
    'page callback' => 'drupal_get_form',
    'page arguments' => array('media_skins_form'),
    'access arguments' => array('access administration pages'),
    'type' => MENU_NORMAL_ITEM,
  );

  return $items;
}

function media_skins_form($form, &$form_state) {

  $form['media_skins_test'] = array(
    '#type' => 'textfield',
    '#title' => t('Test field'),
    '#size' => 20,
    '#maxlength' => 20,
    '#description' => t('Test info for the field.'),
    '#required' => FALSE,
    '#default_value' => variable_get('media_skins_test', 10),
  );
  return system_settings_form($form);
}

function media_skins_form_submit($form, $form_state) {
    variable_set('media_skins_test', $edit['demo_number']);

}

in devel/php you can see the results of the textfield

$test = variable_get('media_skins_test'); dpm($test);