This is a quick thing. When you build an app in PhoneGap Build, it asks for a keystore file. So, how do you get one? Here is the command for on the mac.
keytool -genkey -v -keystore [Name for the file].keystore -alias [Name for the app] -keyalg RSA -keysize 2048 -validity 10000
Remember to replace [Name for the file] and [Name for the app]. After that, just follow the prompts.