The most important thing about working in digital art seems to be very obvious but is often overlooked. It's a form of art and it should also be treated this way. This is something that you should have in mind when working on 3D game models or even concept art, because things such as color and composition will always be part of your work, even if not addressed explicitly.
This doesn't mean that you have to be the world's best painter to start in game art, but just think of your work as art, and also apply the rules of art to it.
Although job positions in game production include the term "artist", for example "concept artist" or "character artist", it is still a field that combines technical and artistic knowledge, and sometimes one outweighs the other.
ZBrush is a very good example of how creating digital art becomes less technical, so more and more people are able to give it a try. As this becomes easier and easier, the real distinction between digital artists will lie in their artistic capability, not their technical knowledge.
If you already have some experience in drawing or painting or even sculpting, it will be even easier for you to use ZBrush. I personally believe that if you can draw it, you can also sculpt it in ZBrush, because both mediums have so much in common.
Like every skill, creating art requires practice. We'll address decisions made out of artistic reasons in this book, so you can decide if you follow them, or alter the examples to your own imagination. Because at the end of the day, it all comes down to being able to give reasons for your decisions.