Modeling is the process of creating specific shapes that describe an object. Through the use of primitive shapes, curves, point, polygons and NURBs, you create what you need (or find it online). Modeling is a true art and science, and it helps to have a good idea of what you need to create before you start the process. If you know how to draw, you can visualize your design from several angles to help determine the best way to build it, as you have numerous options available to do so. Only practive and experience will help you to quickly determine your best choice.
There are many modeling tools and techniques that you can employ within Cinema 4D. Some prefer to block out the basic volume of the design with primitive shapes. You can work quickly this way, and get a good feel for what you need to refine. You can either rebuild what you need at that point, or edit the shapes and work with the various tools to resolve the design.
Other techniques include box modeling, where you start with a cube of several sections, and push, pull, add/remove edges, extrude and so on to get your design to where it needs to be. In many cases, you would add a HyperNURBS tag to the shape to add more detail (though at a loss of some control), and this is common in character modeling.
When modeling vehicles, you can start with blueprints in each view, and build curves in 3D space which correspond to the countours of the vehicle. You can then use extrudes and surface tools to add the skin, and the other tools to further refine the results.
Some modeling techniques are procedural, meaning they are driven by computer code. MoGraph is one of the best illustrations of this approach.