A web service is a collection of web methods. A web site is a set of web pages. You can't just publish a web site as a web service.
You need to define the tasks that need to be performed by some software that calls the web service (known as the service consumer). Once you have those tasks defined, you can construct the necessary web methods to perform those tasks. Generally you will have logic in your web site's pages that performs tasks that are driven by the user interface of the site. This logic needs to be extracted into it's logical areas and placed into the web methods that will be called by the consumer.