As others said learning stl and getting a look at boost is probably a good idea. For Gui / rich clients, Gtk/QT/wx. But you asked for something to see on screen, you could look for graphics library, often used to make games. opengl/DirectX for low level, a bit more high sdl/sfml or go for full engines ogre3d...
There is a lot of c++ libraries and as a self learning person, you will be guided by tutorials, you'll find on internet, so choose something you are interested in, something that you can run on the target and development device (portability), and something widely supported.
In each case, you must compare each libraries to see wich one is the easiest to fullfill your goal, there is no global answer. In the case of Gui widget for example, i advise you to try at least 2 and see wich one better fill your need. try to do a demo application with Qt and gtk for example, see how thoses library are differents and weight your needs.