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/.
You can drop by on the IRC channel #nixos on irc.freenode.net to chat with the developers or other users.
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.
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There are several other manuals: