Your an Artist and an architect...so the best place to start is in a sketchbook, draw out your ideas from different views and think about measurements. Once that stage is done look to the software! this is as personal as a sketchbook. There are many out there that will export to STL for the printer to run off. How complex and what prior knowledge/ skills do you have. There are different price brackets to consider, as well as the operating systems you are familiar with. If its Mac look at Autodesk and the free versions available then work your way up from there.