Well I would recommend that the first thing you do is to create a "game design document" and take many choices regarding the development.
Fundamental points:
-Game overview: Who is the MC? what is he doing? relationships with other characters?
-Fetishes and mechanics: Overall think about what fetishes will you have on the game and how important will it be, it isn't the same having an ocassional scene about a fetish than being a main fetish in the game, overall if the fetish is important enough it will require mechanics related to it.
-Characters: Main characters in the game, it is better to be a complete list, at the very least with the starting characters that you interact with, overall it's important to write their personality and how they act and their relationship towards the MC or other characters is.
-Lore: Basically world development, if there is magic then you have to write about it, monsters, gods or anything else, anything that goes outside our worldview should be written and introduced slowly during the game.
-Quests and interactions: Basically how do you interact with other characters and what happens in the world.
At the start it's important to choose exactly what is the game you are trying to make, what do you want to accomplish?, overall it's important that you enjoy what you do and that you have fun.
