I want users to be able to sign up to a newsletter, and have them submit their email address into a database, how would I do this?
Create your form with a text box with a name of email, then a button with the name submit, using GET. Then $submit = $_GET['submit'], and $email = $_GET['page']. Then check for $submit, if it is found then do "INSERT INTO table_name (email) VALUES ('".$email."')"
if you don't want to deal with the coding on your own, you can use ready-made script that can generate subscription forms (that generates html code and all you do is insert it on your website) and sends newsletters themselves. newsletter.pro, for example, does such job