How do I send an email containing the user password? I'm implementing a function that send the password to users who forgot it.

This is the code I developed so far.

function my_module_lost_password($email) {
  global $language;

  $response = array();
  $account = user_load_by_mail($email);
  $params = array(
    'subject' =&gt; "hi",
    'body' =&gt; "hello",
  );
  $from = "The_admin@gmail.com";
  $to = $email;
 drupal_mail("???", "???", $to, $language, $params, $from); 


}

This function is called by a POST query (web service). 

How do I use drupal_mail() to send a mail to users containing the password from the users table?
First thing, you cannot send the password. You can only send a reset/one-time-login link. Passwords in Drupal are a one-way street, they can never be retrieved. To send the link is pretty easy. Use _user_mail_notify()

_user_mail_notify('password_reset', $account);