3D modeling takes a little bit of practice, and time. There are quite a few open source 3D modelling software available that can to some degree also render your created objects nicely. Maybe have a look at Google SketchUp or Blender, both have a great community and you can find lots of video tutorials and blogs online.
Alternatively, Rhino could do the job too, which is a commercial software. Have a look at Rhino+Grasshoppers PlugIn for parametric modelling.