With nothing selected, apply the Bounding Box preset from the Effects & Presets panel.
Parent the bounding box to the layer you would like to create a bounding box around.
Under the Bounding Box effect you can customize the appearance of the bounding box.
When you initially add a bounding box there will be an expression error until you parent it to another layer.
You can create a bounding box around a pre-comp. Or you can use Get Rect Pro to create a rectangle around multiple layers, then create a bounding box around that rectangle.