Since the new version ZBrush 4R8 was released they started another free 45 day Trial where you can download and test ZBrush and all of its glorious functions for free.
Here is a link to the 45 Day Free Trial
If you are unfamiliar with the Program you can also view tutorials for beginners up to advanced completely free on the Pixologic Homepage.
Here you can find Tutorials
However if you want to jump right into it and follow a complete tutorial online i can recommend this tutorial by Steve James: Steve James Tuorial just follow the parts...
Create your own n stop copying, crap DAZ/Poser models .