Open a new document, and create a new layer.

Using the Rectangular Marquee Tool draw a square selection.

Now select the Angle Gradient Tool and fill the selection using white as the foreground and black as the background.

Now go to Select> Modify> Contract and contract the selection by 2 pixels, and fill that selection with dark grey, and press (Ctrl+D) to deselect.

Once again select the Rectangular Marquee and draw a selection on your image as shown.

Now press the Shift+Ctrl+Alt key together and using your right arrow key drag the image towards the right.

You can now use the Layer> Effects to give various effects to your button.