If you're looking to learn Nix start-to-end, there manuals for Nix and NixOS, both are good.

This is an extensive study at trying to 'understand' Nix, because is does so many things differently: https://ianthehenry.com/posts/how-to-learn-nix/

If you're just looking to start building your system, follow NixOS manual up until you have a running system, then pace yourself as you will. You can also setup Nix on existing Linux distro or macOS and practice the ropes before plunging into NixOS.