I have recentpy chose to update my training material for the PhoneGap library and add a short coverage for using PhoneGap cloud based tool at build.phonegap.com.
The first step would be creating an Adobe ID account. Once created you can proceed and start using this tool.
PhoneGap Build currently supports the iOS, Android and WindowsPhone platforms. In order to get a ready to use application for iOS there are few more steps.
PhoneGap Build can alternatively fetches the files from your account on GIT. In order to keep things simple you can easily replace files that already exist on phonegap servers with new versions. There is no need to start again the whole process.
The simplest way to start would be using the code sample you can find at https://github.com/phonegap/phonegap-start/.