I completely understand your situation, it was also hard for me to start and after doing a couple of projects I'm sometimes still spending hours looking for information.
What really helped me at the beginning was a tutorial on how to build a website from scratch which is already outdated but explained greatly how the different things are interrelated. Check it out:
In any case I don't see the point of "converting an existing site into ez publish" - it will be more like using the existing site for the layout (pagelayout.tpl) and then create content classes that match with the contents you want to present. Later you need to override templates (which means you need to define which template is shown when you click on a menu element of the section "Feedback", for example).
