Start with a short story and not a novel… that way you can finish it, and it will feel good.
As for structure, for me it really helps to think of the game infour different steps since I can’t jump back and forth between different brainspaces.
1. Structure. Plot out the game/choices/chapters on paper so you can draw arrows between the scenes and figure out where everything goes. This is important if you want to finish, otherwise it might go off the rails somewhere down the line. If it looks bare, don’t worry. It will most likely get twice that size as you start writing and figure out more choices that makes sense. This is the fun step, and try really hard not to make your first game longer than a single comic issue or short story. It is easy to go big, but it will be harder in the next step.
2. Coding. Write the actual code of the games, with simple sentences what will go in the flavor text. This way it is easier to see the code, and you get to figure out what kind of stats you need to keep track of. It will be hard and annoying at first, but once you have understood choicescript it will be easy. You don’t need to know everything, just a few basic commands and you are set to go. There are many good tutorials on the site.
3. Writing. Now it is time to switch into writing mode. You don’t have to worry about how to code now, you just go through the game, replacing all those one sentence descriptions with proper scenes and dialog. It is at this stage where it is important to jump back and forth and see so that the end of a scene flows nicely into the next one, and to add page breaks in case things gets congested. It is here that more choices will probably pop up, but be sure not to expand too much.
4: Polish. This is where you go back once more and polish everything, fills out routes that looks bare, adds exposition and makes sure that everything flows nicely.
It is usually during step 3 that the game goes up as a WIP, to get some input as you progress. If you want to.
I can’t stress enough the importance of not thinking too big the first time around. If you have a cool idea for a big game, pick your favorite character in it and try to write a small scene just about them. Because that is the other important thing, you have to like what you are doing, and really want to write about these people/events. Otherwise nothing will work and be fun.