It is all in the manual. Read it well and only post if you are REALLY stuck, you obviously didnt look very hard.
But I'm feeling generous, and your lucky cause I have a handy wee script ready to go!
//Subject
$subject = "Email subject";
//recipient
$email = "somebody@somewhere.com";
//Message
$message = "
<html>
<head>
<title>Title</title>
</head>
<body>
Hello world!
</body>
</html>
";
//To send HTML mail, you can set the Content-type header.
$headers = "MIME-Version: 1.0\r\n";
$headers .= "Content-type: text/html; charset=iso-8859-1\r\n";
//additional headers
$headers .= "From: Andrew Smith <somebody@somewhere.com>\r\n";
//and now mail it
mail($email, $subject, $message, $headers);