There are a lot of my college friends that open game studios now, some are quite successful (e.g. Toge Productions, the maker of Infectonator series).
Ideas or concept is the first requirement. Then choosing the genre (WW2, steampunk, anime, horror, medieval, etc) and the type (flight sim, racing, RPG, FPS, RTS, MOBA, etc).
There is a programming aspect of it, coding your logic into it. Which means, you should choose your game engine.
Then there is creating the game assets, the artworks like sprites, 3D models, textures, sounds, which requires specific skills. You can always do it all by yourself. The quantity, quality, time, effort, all are tradeoffs.
Lastly, testing it. As painful as it may seems, you have to play the game quite repetitively, reproduce the weird events that might not occur in slightly different scenario.