You have a submit.php page (or whatever you want to call it) with the code to send the email.
You need to post the variables from your form.
Create a subject variable with your subject in it.
Create a message variable with your message in it.
Then you add the headers, and use the mail script.
Here's an example of a script:
<?php
$name = $_POST['name'];
$email = $_POST['email'];
$content = $_POST['message'];
$subject = 'Your Subject Goes Here';
$message = '
<html>
<head>
<title>Email Response</title>
</head>
<body>
From: '.$name.'<br>
Message: '.$message.'
</body>
</html>';
// To send HTML mail, the Content-type header must be set
$headers = 'MIME-Version: 1.0' . "\r\n";
$headers .= 'Content-type: text/html; charset=iso-8859-1' . "\r\n";
// Additional headers
$headers .= 'From: '.$email.' <'.$email.'>' . "\r\n";
// Mail it
mail($email, $subject, $message, $headers);
?>
Hope this is helpful!