Everyone has their own process, but I think it’s fine to just start with a simple idea that’s not yet a game, jump in, and experiment around, especially if you’re just starting out. I’m not an expert by any means, but I managed to release a small game of my own recently, and all I had at the start was a test where I could knock over a box by throwing a ball. Everything else that eventually became the game were ideas that I had later while playing around and showing it to other people.