[support] How a module can offer its own webservice ?

Hello, I built my first module which stores data in its own tables using db_query(...). Now, i'd like to manage thoses tables through a webservice which would be a part of my module but i'm wondering how to achieve it. As it is my first module i'm not really familiar with the API so how could i integrate a uri in drupal site which would lead to a php5 web service and how could i get access to db_query() in that script ? I thought about : http://localhost/drupal/sites/default/modules/myfirstmodule/service.php Or maybe is there a special hook in modules that does not lead to a drupal page with theming etc. Until now i only could create blocks. If anybody has hints, i would appreciate. Thanks, Eric