First, in your terminal, we'll generate a private signing key:
$ keytool -genkey -v -keystore friends-release-key.keystore -alias friends -key-alias -keyalg RSA-keysize 2048 -validity 10000
You'll set a password for your keystore and key. Your release key will be labeled friends-release-key, unless you choose otherwise.
The validity flag in this command sets the validity of the key for x number of days. In this case, it's 10000, due to our command.
This keystore file will be located at the directory in which your terminal prompt was when you ran the command. Move it over to android/app in your Friends app repo.