There are many 3D software in the market, as long as it can export .stl format and it is good for printing.
These are a few free 3D software, choose the one you think is right for yourself:
1. Google Sketchup
If you are using Google Sketchup, install this free DXF/STL Exporter plugin to export your model to .stl format.
For those who is new to 3D software, we recommend Google Sketchup. You can easily find many tutorial o Youtube.