The main aim is to create an optical illusion that looks like its on a ledge, it's not easy to get the perspective right but I have written a brief tut for you to follow.
It's by no means an all encompassing tutorial but it should give you some ideas to be going on with.
Set the background colour (in this case I am recreating webdesign.com's header so will use their colour scheme)
Set up your grid to 10px x 10px and make sure that you are snapping to the grid.
Using the Rectangle Shape tool (not the marquee tool), draw out your first main rectangle.
Now draw out the shelf. I have done it 20px high but this can be changed later.
Next count 20px in and do the top most rectangle
Grab the direct selection tool and select the top left point of the ledge layer
Drag the point in to meet the bottom of the other rectangle above it. Do the same on the other side!
This gives you the basic shape so now we need to make it look 3D.
Using the marquee tool, drag out a shape 10px high and the width of your canvas. Create a new layer and fill the shape with white.
Again using the marquee tool, create a rectangle from the top of your canvas down to where the shelf rectangle starts.
Grab the Gradient Fill tool, create a new layer and set the background to black and foreground to transparent
Click and drag so that the gradient fills the bottom half of the marquee rectangle
Deselect that layer and drop the opacity down to around 30%
Go to Filter > Blur > Gaussian Blur and set it to 2
Back to the highlight layer. Go to Filter > Blur > Gaussian Blur and set it to 3.
change the Layer Blending mode to Overlay and drop the opacity down to around 30%
Finally nudge the highlight layer down so it sits below the ledge and you should have something like:
PSD file attached.