Create a square with the marquee tool on a new layer. Make sure it is a perfect square by holding down the shift button as you draw it. Fill the square with a dark grey.
Duplicate the square layer and move it to the left of original square. Use color overlay with a lighter grey and select about 3/4 of the square with the rectangular marquee and delete it.
Duplicate the left side and go to edit>transform>rotate 90 degrees CW. Use color overlay with an even lighter grey and move the new layer to the top of the original square.
Click on the top layer and go to edit>transform>perspective. Change the perspective until it is leaning about 45 degrees to the left. Make the side layer lean about 45 degrees to the top. Adjust the original square layer a bit. If you're not familiar with changing perspective, you may have to play around a bit to get the right effect.
Merge the three layers together by linking all three and pressing ctrl-e. Then ctrl-click the new layer to make a selection around the cube