Write down the step to create:
a. drop shadow
b. clipping mask

Steps in creating a drop shadow:
 
Step 1: Select an object or image.
 Using   the selection tool of your choice(Lasso Tool,Quick Selection Tool, Pen Tool etc...) draw  a selection on your image. Then, when you're done you should see an outline around it.
 
Step 2: Copy the Selection to a New Layer
  At the image selected, press Crtl+J(Win)/ Command+J(Mac) on your keyboard to quickly copy to the selection to a new layer. But if we look at the Layers Panel, we see the Photoshop  has copied  the selection in to a new layer  "Layer 1" above the background layer.

Step 3: Add Extra Canvass Space if Needed 
  Depends on you if you want this step but in your case I have to teach it on how to add some extra canvass space  to make room for the shadow. The easiest way for this is the Crop Tool. You will found it in the Tools Panel. For the fullscreen mode press the letter F on your keyboard.Click Crop Tool for the active mode, use any color of your choice then use your mouse to drag from left top to right of the bottom right corner. When you released it a cropped border appears all around the image, with handles (little square) all of the four side of the corner.Then press ENTER (win)/ RETURN (mac) to allow the crop. Press F on your keyboard for a couple of times until you reach your document window view. 

Step 4: Add a New Blank below Layer 1 
  Hold down your  Crtl/ Command  key and click on the New Layer icon at the bottom of the layers panel. Which places it between layer 1 and the background layer. 

Step 5: Draw a Gradient on the New Layer