Contribute to some other game that already exists if possible. that is the power of open source - when many people over many years come together to work on all the tedious details to make a complex polished game free from market pressure. When you do many choices have already been made for you, so they can tell you where to start. Want to add a new puzzle, put a picture on some wall, or whatever - they will tell you how to do that.

If nothing is like your idea you can start from scratch. Likewise sometimes the existing people involved in the game are jerks and you need to start over or fork - but are you sure it is them and not you. These are not ideal cases though and should be avoided. Much better to work with others if possible.

If you start from scratch you should be thinking about what game engine to write in. You can write a game from scratch in raw code, but in general step one is picking your engine and then living with the limits of that engine.

I sure can contribute some whitespace fixing commits. When do I start?

I mean first, I’d probably ask you to check out what exists.

RTS: Beyond All Reason

Roleplaying: Space Station 14 (Space Station 13 “sequel” decouples from Beyond Launcher and apparently easier to contribute and extend)

Roguelike: Dungeon Crawl Stone Soup

FPS: Unvanquished (I have never played or seen this one)

Theres much more here:
List of Open Source Games

It may feel good to join something already moving than trying to get people to start over on something new.