First of all, I have no experience in working with ONVIF at all. I got myself a scholarship at a company and was asked to work with it (to control some cameras and get photos fr
So, you have to start with gSOAP.
Pelco has a guide to get started with gSOAP, it is for windows but it is valid for Linux as well. As you start learning gSOAP, check the last FAQ of gSOAP about the namespaces.
Finally, on the ONVIF website there's an Application Programmers Guide, which is old but still interesting. ONVIF has a forum for all people willing to learn the protocol. It does not require you to be member to access it, you can join it for free and check all the questions there.