I agree with Axi5. As a beginner you should learn to model these things first before using pre-built meshes. For both practice and experience. It's like the difference between a 'script-kiddie' and a real programmer.
Depends on the nature of the challenge. If you're focused purely on concepting a design in zbrush, most people wouldn't fault you for using a base mesh you didn't make, or had prepared beforehand. The exception there is that you're using a generic human shape, and not something like a premade half human/half spider kind of…