Here are some starting points for your journey with Zotonic.
When you want to install Zotonic, please read the file "doc/INSTALL".
When you want to proxy Zotonic using nginx, then check also the file "doc/INSTALL.nginx" for an example site definition. Similarly, doc/INSTALL.varnish shows how to put Zotonic behind the varnish http frontend.
When you want to read the source, then start with "src/zotonic_sup.erl" and work your way down using the servers defined in there. After you have an idea what is what. Then check the modules to see how they implement all the functionality, start with "mod_base".
Sites live in priv/sites/*. Their templates are in organized in subfolders. For the templates of the default site, look in priv/sites/default/templates.
Another useful source of information is the module "src/install/z_install.erl", this erlang module contains the base SQL datamodel. Check also "src/install/z_install_data.erl" as it defines a basic set of predicates, categories and whatnot.
Thank you and have fun with Zotonic.
Tim Benniks, Arjan Scherpenisse & Marc Worrell