Trimble SketchUp is a free 3D modeler.

Your SketchUp file must be one continuous, solid object. The printer must be able to clearly distinguish between the inside and outside of the object it is printing. There cannot be holes or gaps in the model, because the 3D printer will not be able to tell what is inside the object and what is outside. There are tools that can help identify an close holes and gaps, but the majority of these areas should be found and closed within the original CAD model for best results.