Load a picture and look for the layers pallete. If you don't see it hit F7 and it will blink on/off.
1) Now copy the picture into a new layer (Layer -> Duplicate Layer)
2) Expand the canvas (Image -> Expand Canvas) do it either by Percent or by pixels to make the background the size you want. Set the expand color to White
3) Go over to the layer menu and select the layer created in step 1.
Right click on it and select "Blending Options". You will find a check box for "Drop shadow". Turn it on and then muck around with the settings until you like it.
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4) Go pick up a book by Scott Kelby (Photoshop for Digital Photographers). If you don't know how to use layers you are missing a lot of the fun and power of Photoshop.