Step 1.
Open an existing document, or start a new one of any size. I'm using 500 x 300 with a simple grid background.
Step 2.
Create a new layer and using the rectangular marquee tool make a selection.
Fill this selection with white (#FFFFFF)
Step 3.
Double click this layer, and apply the following blending options:
Gradient Overlay
Because it's difficult to see at this point, I've added a "lime green" highlight around the result, here's how yours should look (less the green highlight of course):
Step 4.
Create a new layer. Now set your foreground color to: #4A4A4A and get out the 'Pencil Tool'. Draw a 1 pixel line across the top of your box, and down the left side as pictured:
Step 5.
Finally set your foreground color to: #131313 (dark dark grey) and again with the Pencil Tool, draw a 1 pixel line along the bottom of your box, and down the right side, as pictured:
And that's all there is to it. All you need to do now is add text, and icons and you're finished. Here's what I ended up with after fooling around with it for a few minutes:
