Look, these two words are usually used interchangeably by non-native speakers. But there are subtle differences depending on the context.
From what I see, ready is much more immediate than prepared, and we use it when talking about something expected to occur very soon. If we are prepared, we have gotten ready in advance.
For instance, we can be prepared for an Ebola outbreak in Brazil - but one is extremely unlikely to happen. Or we can say: I'm ready shortly before I leave, but speak of preparing during the time leading up to it.
When it comes to vocabulary, certain subtleties are hard to grasp unless you have been in touch (immersed) with the language on a daily basis for several years - that is the case for native speakers. Non-native speakers like myself, every now and then, struggle with such questions.