Sort of expanding on Pior's comment, I think having a really well defined goal helps. Do some projects where you have strong references or concept art. Sculpt a real person, like an actor, so you know when something looks off. You'll learn the software by solving little problems as you encounter them. It's basically sculpt -> sculpt -> research a problem -> sculpt (and so on).
Replies
https://pixologic.com/zclassroom/
and this was my favorite book for zbrush
https://shop.3dtotal.com/zbrush-character-sculpting-volume-1.html
The easy part is to actually learn how to use the software...