One of the original starting points for the Flat Earth subject matter were the Flat Earth Clues videos done by Mark Sargent. Mark has create 20 or so videos starting back in February of 2015. I have found these videos as a soft entry into the subject and don’t really prove anything but more lead you to the questions and information that will lead you to consider the possibility that the earth is indeed flat. Here is a link to that video playlist.
Another one of the original Flat Earth stewards is Eric Dubay. He has two key videos. The first is 200 Proofs Earth is not a Spinning Ball. The second is and audiobook read on YouTube, The Flat Earth Conspiracy.