Software for designing 3D models abound on the market.
Some of them are easy to learn and use  and others more complex, with prices spanning from free to higher budget.

A non-exhaustive list can be found here:
http://en.wikipedia.org/wiki/List_of_3D_computer_graphics_software

It is also possible to create 3D models by scanning.
3D scanning allows you to scan objects from multiple angles to capture the apparent and hidden parts of your object.
The collected data can be combined to create the best possible 3D model.
More information can be found here :
http://en.wikipedia.org/wiki/3D_scanner