How are visual novels structured?

I want to make one but im clueless to how they are structured.

Well not all visual novels are structured the same, VNs of varying genres have notably different structures. A horror VN might focus on world building before characters for instance (which isn't always true and probable usually not). Kara no Shoujo (18+ for sure), "999: 9 hours.." (blah blah), and Phenomeno are some of my favorites, they have completely different structuring I would say. I recommend just playing through a couple visual novels if you want good references.

It's pretty much structured like a book, except somewhere down the middle you get to make choices which may affect the story (eg. date sim aspect, bad endings, different route).

However, like normal books, you can pretty much structure them however you want. The vn Ever17 is one of the few ones I've played which actually makes full use of the visual novel platform to help with the story telling. People who have played it will know what I'm on. (Coco Route mind = blown)

And yeah, the best way to understand the platform is just to play some of the games.

Recommendations: Steins;Gate (different endings, story doesn't really change)

                                Fate/Stay Night (18+) (3 different routes which are like standalone stories, text covers whole screen)

                                Kara no Shoujo (18+) (interactive 'investigation' and 'exploration' system)

                                Kamidori Alchemy Meister (18+) (pretty much a hybrid RPG but still a visual novel because there's somewhat of a story/routes.)

The 18+ all have H scenes, though they don't really do anything to help advance the story...

Ever17 and If my Heart had Wings are all-ages but pretty long and drawn out