VistA's built around FileMan and FileMan has over 2000 first class file types - over 5000 if you count subfiles. Where to start?
Maybe with Patients or its partner, IHS Patient. Patient data clusters around these two. Maybe see who's been in Bed 2 or who's on the Medical Staff or what Wards does your institution have or HL7 TCP Links or ...? Maybe you only want meta data? The Schema, every package or ...
The best way to learn FileMan is go for a walk. Into its files, about its files. Just ramble ...