via Anaconda Navigator
To install a package from Anaconda Navigator, select the Environment section on the left pane. The default environment should be base (root) (in case select a different environment).
Check that the filter is set to All and not only to Installed. Search for the package you want to install, select it and hit Apply on the lower right corner. The package will be installed with all the related dependencies.
If you know the package is available only in a certain channel, press the Channels button, select Add and insert the name of the channel you want to add. Indeed, many packages and dependencies are available through the conda-forge channel. It is recommended that you add it to your environment.
If you cannot find the package in any channel, then the package is not available through conda and you need to install it with pip. See the next section.
via Anaconda Prompt
You can install a package launching your terminal (or Anaconda Prompt as Administrator if you are on Windows) and typing:
$ conda install [packagename]
Or in case you need to install it from a particular channel:
$ conda install -c [channelname] [packagename]
To add a channel to your environment, so packages are installed directly from it, just type:
$ conda config --add channels [channelname]
In case the package is not available through conda, you can install it with pip:
$ pip install [packagename]