I always used to think that this sort of meant ‘Johnny be a good boy’, in it’s general sense, as sometimes kids are told at home: “Johnny – don’t be troublesome, be a good sincere boy.”
But after having listened to it several times, I suddenly realized that this in fact means: “Johnny, be awesome. Irrespective of your background, and your knowledge of the world – you can be awesome. So go, become!” Since that sudden realization, it has become almost an inspirational song to me!