Now we know why we need a YouTube channel and how to make one, so what do we put on it? Anything you want! (Don't infringe copyrights). Think of your YouTube channel like a blog. What do you put on your blog? Writing advice? Book trailers? Funny articles? Book reviews? (If you have a book out, you might even consider doing a reading).
You can put on these things on your YouTube channel. You're just going to have to get comfortable talking to a camera. Like blogs, the most trafficked YouTube channels are informative or funny. People want to know they're getting something when they come to visit you. I think the most popular author YouTube channel is John Green's, and it's not even always about writing. And you build your following the same way you built your blog. You become part of the reading/writing community. You comment on others' content. You follow channels you like and let them follow back. And you have a whole new platform. A place to reach people who may not be reading you yet.
Do you have a YouTube channel? What do you put on it?