An underrated place to start is with Doom modding. The various ports of Doom engines have grown to have surprizingly rich feature sets these days, and a lot of indie games have been getting made in these engines. It’s easy to get into because you can get started pretty much right away by making mods for existing games.

So for example, you could download the free game FreeDoom, get the UZDoom engine to play it on, the map maker Ultimate Doom Builder to make maps for FreeDoom, SLADE for the other aspects of editing, and then finally the ZDoom wiki should be able to point you toward everything you’ll want to learn to be able to make whatever you’d like.

So you can start learning and producing content very quickly by making maps, learn and develop those maps into more extensive mods, eventually learn and develop those mods so far they become “total conversions” - meaning that it’s still technically a mod, and still would depend on the Freedoom game you’re building on, but has grown extensively enough to replace all assets; and then finally it is possible to convert that total conversion into a full fledged standalone game.

And of course once you have enough experience with this, you can develop standalone games right from the start of a project.