The very best design tools are a stack of plain paper and some sharpened pencils. That said, there are other design tools that you might have fun using:
Once you’ve got your ideas on paper, you can get fancy by downloading and using some digital tools to help you take your design further. A really useful tool (and free) is Google Sketchup. This is a 3D modeling tool that is very easy to use. Be sure to watch their great instructional videos if you are using Sketchup for the first time.
Download a copy of Sketchup here: Google Sketchup.