Dave has an older computer that he wants to turn into a server. Leo recommends Linux, using Debian LTS. It's free.
Dave also is wanting to know about macOS and permissions. Leo says that modern operating systems are designed to be used by multiple users and as such, permissions protect access to your data. With OSX Catalina coming soon, permissions are going to be a more serious affair, with restricting access to some features to only administrators. Leo recommends reading up on permissions. Dave can also drill down on the properties of any file by right-clicking on it and selecting properties.