Consider this another vote for Ubuntu or any of its variants. They're beginner friendly, and established enough that you'll find plenty of resources written specifically for them. Linux Mint is another one I'd recommend for beginners, it's designed to "just work" out of the box and be an easy transition for Windows users.

Then it's just down to using it some. First and foremost, leave Windows installed until you're comfortable with whatever else you end up trying. Whether you partition, or make a bootable USB drive, or even just a VM, use some kind of temporary space for practice. The terminal is a lot less intimidating when you aren't learning in your main environment, you can go break things and see what happens.