Start small. Smaller than you think you need to. How small are you thinking? Think smaller than that.
Don’t start with Mario. Or even Tetris. Start with Pong. Not even two-player Pong. Start with a bouncing ball. Add a single paddle. Go from there.
Don’t think about “how would I make this whole game in Java”- instead, start by breaking the game down into its individual pieces. What is the game made up of? What kind of collision detection does it need? What is at the core of the game? Then start thinking about how you’d implement those individual pieces in Java.
Every version of your game should be playable, even if that play is boring and repetitive. A game has a beginning (title screen), middle (where you play), and end (game over screen). Build those in from the very beginning.
LibGDX is your friend.