You should learn C++ and then get a look on popular GUI libraries. They wraps all platform depended stuff suggesting you unifonm interface.
when you become a programmer you will design core of Program and then someone else other programmers will design the GUI parts !
That depends. Usually it will be like that only in very large companies. And in either case, you will nedd GUI for your own internal application, quick mock-ups to know how it will work and other.
So, when you get better at C++, get familiar with popular libraries (not only GUI)