If you truly want to become good at sculpting, just download the sculpting package and start working at it. Retopology is important, and much of the aftermath of what you do in zBrush you can easily do in external packages and modelling applications. But if you learn sculpting first, you can always go back and learn the technical afterwards. To be honest though, if you want to become a demi god at sculpting people with a head start, you should be studying the human anatomy. 2D skills in the human body are a major plus, but not required.