You might try a sculpting class at your local community college. It'll get you some basic skills, resources to use, and be inexpensive. Talk to the teacher at some point in class about what you'd like to do. If more of your ideas run this way you'll have a good start.

My prototypes usually start out on engineering paper, get a few quick 3D CAD drawings, then get made up in polymer clay as a form for a mold for shell casting. The 3D step is new, and hasn't eliminated the clay stage, where I still catch problems and see new opportunities that the computer doesn't bring out for me.

So I'll make a solid item out of polymer clay, then press that into more clay to make a negative mold. Then I make a shell casting by putting mold release on the negative form, pouring in some liquid acrylic, and wobbling the mold in my hand to let the acrylic build up a shell over the whole surface. There are other ways I use, too, but this is a favorite quick and dirty method.

There are books on plastic casting that teach the basics, but there's nothing like hands on in a class with an instructor, even if all you're doing in the class is learning the basic sculpting skills and tools.