How do you create your own RPG system?

Sorry if this question has been asked a million times, but I’m pretty new to RPGs, so I thought this would be the best place to ask. Anyway, I was planning to run a solo investigative RPG campaign for a friend; the story is already fully fleshed out, but I’d like some advice on how to create my own RPG system. It’s my first time GMing, so I have no idea how to go about designing a system from scratch—I just want it to be really simple (Apologies again, because I know the answer might be obvious, but I really have no idea, I’m really new to RPGs 🥲)