Shadow Effect 3D Text Layer

Because cast shadows only is disabled when you use the C4D rendering engine start with the comp set to Classic 3d, add a 3D text layer, a white 3D solid with the material options set to receive shadows only to the comp and position it to match the geometry in the scene, add a light set to cast shadows, and probably some ambient light to open up the shadows. set up your animation and then select the Camera, lights and text layer, duplicate them, then pre-compose the duplicates, open the pre-comp, select the text layer and press AA to reveal the material options and then set up the extrusion. 

Back in the main comp, you can set the text layer to cast shadows only and make sure the pre-comp is on top and collapse transformations has not been enabled.

A more efficient and better way to do this is to save your Classic 3D comp as a C4D file and do the extrusion in C4D lite. You'll have a lot more options for extruding the text and editing the comp and it will render faster.

There are many ways to add shadow in after effects, but as i understand you i think the easy way for you is to add drop shadow effects to your text layer.