Well I would recommend reading through the build diaries on here to see how folks design and build their robots. Personally, I break out a pencil and paper and get scribbling before committing to anything. I know it is more fashionable to use CAD and if I had some complex components, I would possibly go down that route and then send that component off to be cut professionally.

As far as building a creation, if you have a good design then it becomes easier to build it. Again, the build diaries are often a wealth of information as to what techniques/materials work in this particular field.