In order to operate a Direct Debit collection service you need to have what is called a Service User Number (SUN), and you'll need to talk to your bank about that. The bank must sponsor you before you'll get the SUN, and it may ask you to deposit a substantial sum before it will do that.
An easier answer is to use one of the many third-party Direct Debit facilitators. They'll provide you with a code snippet that you insert into your site page. That will take your customers seamlessly through the process. You'll pay a small transaction charge, but you may find it a worthwhile solution.
Here's one company that offers the service - there are others.