As someone who struggled quite a bit to learn Nix, I’d say start small. Install Nix the package manager on any old distro you’re familiar with and play with standalone Home Manager. Enable one program here and there, have it manage your shells and CLI apps, set environment variables and write files. It’s something you can immediately see the effects of and feels pretty rewarding, so I think it’s a great place to start.

I also happened upon this video series a few days ago and it’s a great resource, easy to parse and walks you through not only the basics but how to look around and investigate in the REPL when you want to figure something out.