How to unchain my program?

Question

I have a simple chat program on java. I build it as maven project in IntelliJ Idea. In the java dir i have 3 packages with 4 classes

|client
|---Client.class
|main
|---Main.class
|---Constants.class
|server
|---Server.class

When i start program I can choose in wich mode to start it (as server or as client).Multi user chat with 1 server to connect.

So, now i need to "unchain" my program. I mean to make program structure as

ConsoleUI---AppLogic---TransportLayer(on sockets)

I need to do it with interfaces.What is it I don't know. Can you help me. What can read such newbiee to get understanding of what's going on, how to realize it in the context of my program...