Can ZBrush be used alone? Or do I have to learn other 3D softwares like Maya or Modo?

That depends on what you want to do.
If you just want to do sculpting, then yes, you can just get ZBrush.
If you want to do more than that, like texturing, animation or actual rendering, you need to have some knowledge in other 3D packages and the packages themselves.
As a 3D package, I would recommend Cinema 4D r14. It is incredibly easy to learn, is just as powerful as the other 3D software packages, and it also has its own sculpting tools, so you do not even need zbrush.