Oh, thanks ^^; If you have an idea, you should think about ways to make it playable for several players. They need to be able to use their own ideas and write their own stories but they want to always have some general direction to move in. Chapters are an easy way to organize things in, but it might also make things a bit predictable. Being the GM and having a character of your own is another way to effectively steer the story enough. If you aren't afraid of daunting texts, perhaps read through the first pages of Pokémon Odyssey: A new beginning and see how Supervegeta does it? Also do check out the last pages of Earth & Sky to see how Quto does. I consider those two very good GMs.
As for what you'll need to think up beforehand and have in your first post of the RP, I always think that less is more. Don't reveal the whole plot and reasons behind everything, but write out enough to let the players know what they will be doing and get them hooked on the idea :3
Uh I feel like I'm rambling xD