First, make a really small game, like Space Invaders, or Breakout, or Pong.
Make the game have a start menu, a high score, and extra lives, so you get through how to keep state within the game.
Second, make a slightly bigger game – here’s where something like an Endless Runner could work, or my favorite game example, Bomberman. (Which, as a bonus, has an obvious multi-player extension.)
Or try re-making your favorite 8-bit-console game – Mario, or Sonic, or Pokemon, or whatever.
Of course, it depends on what kind of games you want to know to make in the end. A visual novel is very different from a platformer, is very different from a first person shooter. You’re almost always going to need some UI, and some character animation, but many other things (level design, camera control, physics …) are very different depending on where you go.