3D models can be generated using a 3D scanning Apps. An example app is Trnio which allows you to take a series of photos of your product and it generates the model for you. There are others available for use with your phone and with specialised 3D laser scanning cameras.
Alternatively if you would like to draw your models we have found that Blender and SketchUp which are both available for free are great and easy to learn options for users just getting into 3D modelling. Below you will find a list of useful links and tutorials in using SketchUp.