Assuming the server is already set up to have either SMTP and/or sendmail, then most likely what you are looking for is the mail() function.
Basic usage for that would be something like this:
mail( recipient@email.com , The subject , A short or long message );
when sending a long message, use the wordwrap() function, to ensure that the message is broken displayed properly by the client.
$message = wordwrap($message, 70);
Of course, if this does not work, then you may need to contact your server administrator to make sure that everything is correctly setup to allow you to send email.