I need a basic script to add to my website. It should utilize user names and passwords. Once the customer pays for the service, he/she should be able to set up a user account that he/she can use to login. Does this kind of script exist?
You can use Google to find free examples of such scripts. Also you need to pay attention to what kind of script is the most suitable for your site (JavaScript, PHP, etc.) and how to incorporate that script to your website.