this is not a full tutorial and you have to make some changes to make it look exactly like your photo. this is just a base you can build onto .
Make a blue square on a layer. (with a gradient)
new layer, make a grey square so as the first on. move this layer beneath your blue square layer.
new layer on top of the blue square. use the text tool and write "Ps"
select the 2 letters and go to the blue square layer and delete over the 2 letters. now hide you text layer.
go to the blue square layer and select the square and the deleted letters.
go to effects > objects > Drop Shadow*
and then twist the settings to your liking.
*you need the plug-in Drop Shadow, if you dont have you can search for it in the plug-in section.