Sounds like what you are looking for is in the Solid spec and the related LDP (Linked Data Platform) spec.
Unless… you are wanting to know what the structure of the pod storage itself should be? The latter isn’t specified AFAIK (though in the past there were some guidelines, I think these are no longer current). So you will find some conventions, such as a top level folder called ‘public’, but that is not mandatory.
One way to explore what the structure of a pod might be would be to look at example profiles. A profile is an RDF resource which says things about a particular entity, referred to by its WebID. The profile is the document retrieved at the URL of the WebID, and you find information about the entity being described by de-referencing the ‘#’ fragment part of the WebID.
So if my WebID is ‘https://thewebalyst.solid.community#me’ you’ll find a profile document (in RDF) at https://thewebalyst.solid.community and information about me at the node in the document referred to by the ‘#me’. Along with information about me, there may be information about where I store my data is stored. You may or may not have access to that - that will depend on what permissions I have set for the storage in my pod.
Hope that helps a bit!