Always worth starting with the official Moodle docs - https://docs.moodle.org/en/Using_web_services
There is a list of the standard core webservices here: https://docs.moodle.org/dev/Web_service_API_functions
(You can also get a list of webservices inside your Moodle install by ticking the 'generate documentation' link on the 'enable protocols' page).
If you cannot find an existing webservice that does what you want, then you may need a developer to create a webservice, as per the instructions here: https://docs.moodle.org/dev/Adding_a_web_service_to_a_plugin
The recommended place to find a suitable developer is through the Moodle Partners network.