Just talking about 3D layers would take us all day because it is a huge topic. However, if you are just getting into the world of motion graphics, there are some basic tools you should know to get started, especially if you want to learn how to turn 2D images into 3D animations in After Effects.
First, go to the Toolbar, and see how, right next to the enabled buttons in the home area, there are three other options that are grayed out or turned off. That set of buttons is related to the 3D shapes you have in your project, which means that none of them will be available while working on 2D layers.
Once you have done those three steps and you have your new 3D layer, you will notice that the three hidden features we mentioned previously are now enabled. These are the tools that will help you to transform your 3D objects.
If you are a beginner in the world of After Effects and you are just learning to work with 2D layers, you probably won't need much of these last features.
However, if your goal is to become a top league motion graphics designer, it's good to get familiar with the basics of 3D tools right from the start.