"Sculptris provides an excellent gateway into the exciting world of 3D. It's features are easy to learn, even for someone with no experience in digital art, yet robust enough for creating base models that can then be refined in other applications (...)" - in http://pixologic.com/sculptris/
You can download and use this software for free.
Here are a few tutorials to help you get started:
You may also be interested in creating your own brushes for Sculptris using GIMP:
Have fun, and share your creations :)