Trying to set up a media server for the first time

So I have an older gateway tower I would like to turn into a headless media server, but literally have no idea where to start! I know I need to add some storage to it and install at least a server level os (ubuntu is my main choice), but beyond that I am lost! Would like to be able to stream movies and music throughout the house just don’t know where to start. I have looked at too many “guides” on the subject and all of them seem to exclude how to get the media onto the server drives so it can be streamed…

Any help would be greatly appreciated or at least a direction to go in

I use Open Media Vault and the OMV Extras Plugins for running my media servers.