To "get ready" is an action. You are doing something in order to become ready. For example, if you plan to go out later, you may need to change your clothes in order to "get ready" to go.
To "be ready" is a state. It's not an action. "I am ready" is a description of your current condition: you are prepared. You have already done everything you needed to do in order to get ready, and now you ARE ready. The "getting ready" is finished.
If you tell someone to "get ready," you are telling them to do something. If you tell someone to "be ready," you are telling them what condition you want them to be in. If the person needs to "get ready" in order to "be ready," then the two commands will mean essentially the same thing.
Other adjectives work the same way. To "get tall" is to grow tall or become tall. To "be tall" is a state. It means that you are already tall. You are not becoming tall. You simply are tall. The word "ready" works the same way.