Unless you have a computer which is already toast, I'd recommend grabbing a virtual machine like VirtualBox and install it on there (On Linux there's a thing called QEMU/KVM which is way better, but that doesn't help you now). It's a good way to see what the process is like and try out different flavors without making permanent changes to your machine. If you like it, you can use something like GParted Live to resize partitions and make space to set it up natively. Ideally, the best time to do it is when you run out of space and need to buy a larger hard drive anyway. Then you can just allocate some of your new space for it instead of constricting the filesystems you already have.

Either way, if you have 50-100GB to spare, that should be enough. I have 1732 packages installed on my Gentoo system (an absolutely absurd amount) and it's taking up just over 100GB. Spanning everything from graphics and video production to GIS to 3D modeling and CAD/CAM, ham radio, gaming, flight simulation, virtual machines and emulation, software development, web and database servers, all of the development libraries needed to compile it all from source code and all of the documentation needed to incorporate any of these components into a sudden whimsical project. I have my steam library, music, and personal files stored on another drive, and that will grow as usual for whatever it is that you do.

Personally, I recommend Fedora for beginners. It's very polished and up-to-date. The biggest nuisance about it is you need to enable a 3rd party repository for common non-free graphics drivers and multimedia codecs, but this takes all of five minutes once you have it up and running.