You're not giving enough context to give very good advice. Here's some basic ideas. To design the shape, you can use a computer 3D CAD program, such as Invento

You're not giving enough context to give very good advice. Here's some basic ideas. To design the shape, you can use a computer 3D CAD program, such as Inventor, SolidWorks, etc. Alternatively, you can just use foam, wax, wood, or some other easy to shape material and manually carve a buck (a buck is something that is the same shape as the final product you want, but only used to create a mold). If it's small you may be able to use a rapid prototyping machine (commonly called 3D printers) to create the buck, if you designed it on a computer. To prototype, you can try creating a model out of a different material that's easy to shape, then using that buck to make a mold (perhaps out of silicone). After you have created a mold, you can then use mold release, and mix and pour in silicone to create your prototype of the shape you actually want.