If the user has the following form fields
to_email
subject
message
And you method was POST on your form, then you would use
$to_email = $POST['to_email'];
$subject = $POST['subject'];
$message = $_POST['message'];
To get the from to be from whatever this guys email is, i'm guessing his email is stored in a database, so call it from the database and create a variable out of it. Then use mail() to mail it
mail($to_email, $subject, $message, $headers);
$headers would include whoever the email was from.
Cgraz