The main source of information about installing and using NixOS is the NixOS manual. Inside a NixOS system, you can read the manual by running the command nixos-help, and you can see the list of configuration options by running man configuration.nix. You can also search the list of options online.
Discourse is a forum where we have discussions and help users out with their questions. Visit the page and register with GitHub or email/password: https://discourse.nixos.org/.
The Nix Pills are to convince you to give Nix a try, and to complement the explanations from the more formal documents, in short digestable pieces.
There are several other manuals: