Since the "developer" in "game developer" refers to someone who writes the code, the first thing to learn is how to program a computer. There are many free, online resources to help you do that; but it takes time. How much time depends on you, of course, but there's a lot to learn and most of the basic stuff won't look anything like programming any game you are probably imagining.
Something to consider: Why do you want to learn game development?
If you want to make money at it, you're going to face some stiff competition. To even get an interview for an entry-level programming at a major game house, you'll need a BS in computer science. To land one, you might need more. For that, make sure your math-science skills are in good shape.
And while you're doing that, be sure to develop programming skills on your own. Universities aren't as good at teaching how to program as they are at teaching how to program better. If you're not interested enough to pursue programming on your own before even applying, think about another career.
But, by all means, check out some free courses to see if you like it. Even if you don't find that it's a career fit, coding makes an interesting hobby.