Open your image file. This will be the "canvas." In the Layers palette it will show as Background Layer.
Next, open the file containing the object which you wish to place on the "canvas" and use one of the selection tools to select it. Then go to Edit>copy to place it on the clipboard. Go back to the original image (canvas), then Edit>paste. Activate the move tool and position the object. At this point you should have 2 layers. If you wish to populate the canvas with more objects, continue on. If this is not clear, feel free to repost.