Itch is not a game engine or platform; you cannot create a game directly on Itch.
First, you must create your game from scratch, or use a game engine, and then export it to the web. Once your game is created and working, you can upload it to Itch for distribution.
First, choose a game engine. For example, here's a tutorial on how to create a game with Godot:
Once you've created your game, find a tutorial on how to export it from that engine to Itch:
Example: How to publish a game created with Godot on Itch.io