If you are new to php and need to send emails then PHP offers you many ways.
But you can use the PHP code below, It's simple and does not use yourSMTP.
Create a PHP page sentemail.php
$to = "firstname.lastname@example.org"; // this is your Email address
$from = $_POST['email']; // this is the sender's Email address
$first_name = $_POST['first_name'];
$last_name = $_POST['last_name'];
$subject = "Form submission";
$subject2 = "Copy of your form submission";
$message = $first_name . " " . $last_name . " wrote the following:" . "\n\n" . $_POST['message'];
$message2 = "Here is a copy of your message " . $first_name . "\n\n" . $_POST['message'];
$headers = "From:" . $from;
$headers2 = "From:" . $to;
mail($from,$subject2,$message2,$headers2); // sends a copy of the message to the sender
echo "Mail Sent. Thank you " . $first_name . ", we will contact you shortly.";
// You can also use header('Location: thank_you.php'); to redirect to another page.