C# and C++ are amazing languages, but when using them in Godot there's some caveats. For starters, there's barely any official documentation, let alone fan-made, because the whole engine was designed around GDScript. That also entails that tutorials for C# and C++ are nonexistent. If you absolutely needed to choose between the two languages, I would choose C#, as it has slightly more support than C++ at the moment and there are some bits of the official Godot docs that are very helpful, but if you're thinking you're going to use Godot for a while, it's extremely beneficial to learn GDScript. It's not an elegant solution, I know, but you'll have a lot tougher of a time if you want to use other languages. As for the Godot documentation, I'll link to it at the bottom of the reply. If you don't know a thing about Godot's basic functionality, I HIGHLY recommend going to the "First 2D Game" or "First 3D Game" bits, and then soon after trying to make a small project based upon what you've learned. If you're not a fan of reading, there's also some cool tutorial channels like "GDQuest", "PlayWithFurcifer", and "Godotneers" to name a few. I hope you have a good time with game dev though!
Introduction
Welcome to the official documentation of Godot Engine, the free and open source community-driven 2D and 3D game engine! Behind this mouthful, you will find a powerful yet user-friendly tool that yo...
Godot Engine documentation (docs.godotengine.org)