Orbabro Well. That's a very broad question. As already suggested, a good start would be to boot Godot, and then just think about the most basic stuff and make small prototypes to learn. For example, start with a moving box on a flat floor. Once you've got a left and right movement nailed, then perhaps add the jump.
And you repeat the same approach for more game mechanics.
May I ask what your experience is? I just assumed that you never made a game or similar project before (but I could be totally wrong about that)? If the assumption is correct, then I wouldn't worry about graphics at this point. Just learn and get used to some of the game dev concepts, and if you are new to programming, learn some of the basics.
But in order to give better advice, maybe share a bit about your experience.
