You got Cinema 4D because of its awesome motion graphics capabilities. You have played with the dynamics system. Yes, it is awesome. You launched particles everywhere with the particle systems. Yes, they are fun to manipulate. 3D Text? Yep, no problem there.
But now down to the brass tacks of 3D modeling. Where do you begin? Is there a right and wrong way to go about it? Polygonal modeling or spline modeling? Below are some tips to help you get started thinking about how to approach 3D modeling in Cinema 4D. Unfortunately, there are no shortcuts to become a better 3D modeler. The only promise I can make is that it gets easier with each attempt.
Keep objects parametric for as long as you can before converting to a polygonal object to be as flexible in your modeling as possible. Use the Correction Deformer to get access to points on a parametric object to get more modeling control. Use the Connect Object to solve issues where objects need to be connected but you don't want to run a Connect and Delete Command such as working with shaders. It's possible to model the same object using a parametric shape, splines, or generators.
Save Selection Sets of polygons as you work to quickly recall and modify selections of geometry further in the modeling process. Use Loop Selections to select loops of polygons, points, and edges to add detail in objects.
Learn the tools and commands of modeling. Extrude. Extrude Inner. Brush. Magnet. Split. Disconnect. Too many to list but these are more the common functions you will use when you get started.
Try to always model in QUADS for clean geometry. Avoid N-gons and triangles if at all possible. Weld Points and Dissolve Edges to clean up your mesh. To have a better look at your geometry, turn off the Phong Tag. Instead of extruding polygons, try extruding edges.
ADVANACED TIP: You may have to retopologize your model if the mesh is real heavy. Basically, you are remodeling a heavy mesh with a lower amount of polygons so it's more suitable for animation by laying down a low polygon mesh on top of the high mesh. Chris Korn has a good Cinema 4D retopology overview in this video from Siggraph 2013.
The more reference images you can acquire such as orthographic views, perspective shots and closeups of textures the easier it will be to model. Can’t find anything good to model? Load a model from the Content Browser in to a C4D scene, render out each view for reference and try to remodel it from scratch.
Version your Project out, such as Modelhead_001.c4d and SAVE INCREMENTAL so you can quickly recovering from a mistake. Don't rely on Undos and Reverts. Create a "Hider" Null Object, as mentioned by Rob Garrott in his many Cinema 4D tutorials to place copies of your geometry in before you make big changes to the mesh such as a Connect and Delete command, or make an object editable (C-Shortcut).
Give your scene a basic three point lighting setup and switch it on and off as you render to see how the model is reacting to light in the scene. To further define the model, use a Sculpt Shader supplied with the Studio version to better view the model. I like the brown clay shader.
Model the same object over and over until you can model it without even thinking about it. You will learn from your mistakes real quick. Watch modeling tutorials for ANY 3D program and adapt it to Cinema 4D. 3D modeling programs share many of the same principles.
If you model symmetrically you only have to model half the object. That makes sense, right? But what if you want a more organic model? After you are done modeling with a Symmetry Object, copy it, and make one editable. Now selectively move points and polygons to add a bit of variance to the model.
Start with base meshes of objects and try sculpting a model. The supplied Cinema 4D base meshes can probably be turned into anything if you work on it long enough. Don’t forget you can use the sculpt tools on polygon objects without the Sculpt Tag.