I didn’t know very much about 3d modeling when I started with Zbrush. I do now own Modo, Lightwave and Blender. I spend most of my time in Zbrush and Blender.
I don’t use blender because it’s free. I use it because it’s excellent and the abundance of free Blender tutorials on the web taught me enough about 3d modeling that I’m now confident I could tackle any other modeling program, with some introduction to the interface of the program, of course. But the concepts are the same for all programs. So that’s the hurdle you have to overcome. And the terminology differs from program to program (vertices or points, lines or edges, things like that) You need to learn what functions the product should be able to do and each program which may have it’s own special name for a particular function, but they all do basically the same thing with a few exceptions.
And you will find having a 3d modeling package like blender or Maya available to work along side of Zbrush very useful, so I’d suggest you learn both* at the same time. Sometimes I choose to build a mesh with zspheres, some times its quicker and more accurate to box model the form in Blender and import it to Zbrush for sculpting. Having these options is very handy.
Edit: *By both I mean the 3d package you choose to work with and Zbrush (not both maya and blender)…I don’t think I very was clear there.