They have a getting started guide on their wiki I believe
Once you install it it should generate a config file in /etc/nixos/configuration.nix, the way I did it was reading that and figuring it out from there. (That file is your universal source of truth for your entire system)
You can install packages/software by adding them to the systemPackages block in the file and running sudo nixos-rebuild switch and you can find pretty much anything you need on nixos.org/nixos/packages.html
Also the community is pretty friendly, can generally get your questions answered on lemmy/matrix