This is an impossible question to answer. How do you build a house? You don't need to learn how to make an RPG, you need to learn how to make games, which means learning how to code, how to animate, how to draw/3D model, how to design sound and music, and much more.
You don't "make an RPG", you make a thousand little game systems and components that all come together to make an RPG. That being said, an RPG itself is probably one of the biggest and most complicated game projects you can take on. It is definitely not something anyone can make as their first game.
If you're interested in learning how to make games, you can start by learning to code. You can try the official Godot tutorial as a starting point:
However, this does assume that you're coming to Godot (the game engine) with some basic knowledge of programming. If you find that you're not at the level where you can understand the code, there are some recommendations for some other resources where you can learn how to do that.
Once you begin to feel comfortable using Godot, you should try making some simple games. Can you make Flappy Bird? Asteroids? Doodle Jump? If you can't sit down and make those games, without help, then you're not ready to try taking on something as huge as an RPG.