What is the best interactive platform to learn the fundamentals of the C# language?
I'm looking into coding my own game, what do you recommend or what did you use when first learning how to code C#?
For learning C#, there are some great resources such as Codecademy or w3schools. The most important thing is to get a firm grasp of the basics of syntax and structure and build from there. You can make a good many things, but to make them well you will need to dive a little deeper into concepts such as object oriented programming (Inheritance, Polymorphism, Abstraction, Encapsulation).
YouTube is also a fantastic resource as there are not only professionals who have great content explaining syntax and concepts, but also professors who post their lectures as well.
My final suggestion is Google, which seems obvious, but as you start to ask how to do things, you will search for it and find code snippets; my best suggestion is to not just use code, but understand it. This may take a little more time and a few more questions, but you will be far better off.
You sound like a self-starter, and someone who does well teaching themselves new things. I started my software development career the same way, mostly because I never found good schooling options that fit my style. I would very highly recommend looking at a school like Western Governors University, I took the software developers path there and finished it very quickly because all my prior knowledge and experience let me the course completion tests. In addition to specifically providing courses on C# and also mobile development, having this piece of paper behind you will also open additional doors in the future.
If you are looking for tutorials that are of better quality and are willing to pay a cost for, then I would recommend uDemy. You are able to ask questions directly to the instructor so that provides added value.