How to write web services will depend on what you want your services to offer, and what clients you need to support. The easiest way to provide web services in Rails is to make use of Rails 1.2's RESTful features to provide a REST interface to your application. A good introduction to that can be found at:
http://www.softiesonrails.com/2007/3/28/rest-101-part-1-understanding-resources
If you're wanting to provide a SOAP interface, you'll want to look into ActionWebService.
http://wiki.rubyonrails.org/rails/pages/ActionWebService
James.