I don't know anything about forger, but as for zbrush some tips I'd recommend are looking at the types of artwork that inspire you. The tutorials at the Pixologic website and top picks at Artstation are good places to start.

If you're interested in characters (which your picture suggests) some artists I'd recommend you checking out are Raphael Grassetti, Tsvetomir Giorgiev, Luke Starkie, Kris costa/Antropus, Bruno Camara, Jesse Sandifer, Andrew Higginbottom, Shane Olson etc.

Other things to look into are lighting, rendering and composition basics. Learning as much as you can about anatomy and always using reference for everything.

Lastly practice, practice, practice, use a wacom tablet, don't get overwhelmed by the amazing artwork out there and keep your eyez on the prize! :)