In this example, you are going to make a curl corner of the picture, it is very easy by the help of this example.
Picture: Take a picture to curl the corner of the picture.
Selection: Make selection by the polygonal lasso tool (L key) and press Del key.
Draw a shape: Choose Pen tool (P key) with any color and make this shape as I have done here.
Layer style: Go to Layer menu > Layer style > Apply Gradient overlay.
This is your curled picture.
Recommend the tutorial