Curses, start with curses_client from the examples
GTK, in C, start with gChatClient, or at least read it
GTKMM, in C++, maybe learn how to use XML and build the UI that way
Python, with GTK or with TKinter (one, don't try to use both)
Learn the platform-specific way to make UI on your machine
Make an android or IOS app!
Use ImGUI and make a 3D-rendered interface