Web services

Hi Mario,
imo the best way to make web service not confusing:
1) setup a web service in the Moodle admin: http://docs.moodle.org/23/en/How_to_create_and_enable_a_web_service
2) use a demo client to call the web service you setup (I would advice REST protocol over SOAP with Moodle to start learning): https://github.com/moodlehq/sample-ws-clients/tree/master/PHP-REST / https://github.com/moodlehq/sample-ws-clients/tree/master/PHP-SOAP (your nuSOAP will be pretty similar)
3) read the demo client code
4) install ws template and call it to with a client: https://github.com/moodlehq/moodle-local_wstemplate
5) Read the ws template code.
6) Go through admin docs and dev docs: http://docs.moodle.org/23/en/Web_services / http://docs.moodle.org/dev/Web_services
7) You should now have a better understanding, you can try to specify your needs and implement them in a plugin: http://docs.moodle.org/dev/Adding_a_web_service_to_a_plugin

Don't hesitate to search this forum and Google things as much as you can,
Good luck.