/etc/nixos config file . fill it in and run nixos rebuild switch and that becomes your new default system.
https://search.nixos.org/packages
This link helps you find packages, and the options give you the command to add to your config, install it adhoc or install into the current session.
If you have a broadcom WiFi card you may need to add a few lines to your hardware config, but normally you don't touch it.