That's pretty vague, but I'm going to assume that you're trying to setup a newsletter type system.
It would be pretty simple to do. You'll obviously need to store the users email address in the DB. Then when you want to send out the newsletter simply setup a script that loops through all of the records in the db that have chosen to opt-in and then use the mail() function to send them the email.
Now, that part you may get, you might be confused about the actual mail() function. Basically you'll want to get all of the text you want to be in the body of the email and assign it to a var, then pass that as one of the args to the function.
Something like this:
mail($users_email_addy, $subject, $message_body, $additional_headers);
In order for this to work your system will need to be configured correctly to send email. Read this: http://php.net/manual/en/ref.mail.php
