Small AJAX/PHP/MySQL Script Needed

Hi I have a site that needs a very quick script and I don't know AJAX. Basically on the side of every page on my site I will have an input field where a user can input their email address and click submit (to subscribe to my mailing list). When the user clicks submit their email address must be entered into a table in my database. There are many tables in my database so the email must be inserted into a table based on a cookie that is already set. The email address MUST be submitted without refreshing the page and when the user clicks submit, the following should happen: If the email address already exists in the table a message should appear saying something like "Email already exists" and allow the user to try again. If they enter an email address, and it doesn't already exist in the database, it should be inserted into the correct table and a message should replace the form saying a SUCCESS message. If the user leaves the field blank and clicks submit, it should not be submitted and a message should appear "Please enter your email address".