We will not program your assignments for you. You have to show us some effort before we can help you. Post your code, we can help diagnose some problems. But don't come here expecting us to answer questions without any effort on your part.
To get started, ignore the Java part completely. Just think about what you need to do in order to compute prime numbers. Come up with an "algorithm" of how to do it in your mind.
Then re-write the steps you took in your mind as Java statements. Make this a method in a class (perhaps public static void main). Iterate through the steps enough times until you have 15 of them.