Go to the Toolbars, and use the Gradient Tool.
In the top toolbar, you will also see the options that you can change for your mask. How it looks, the colors and the transparency.
Next, you will click on your layer mask in the Layers Palette, so you know you are going to create your mask in the correct location.
Now, click on the bottom of your picture in the middle and hold your mouse. Slowly, drag the cursor upward, diagonal, or whichever way you want the fade to show. Stop where you want the fade to end and let the mouse button go. A faded layer mask - complete.
First, here is my original picture:
For this example, I used the faint grey gradient in the lower right corner. There is a slight darkness that comes from the lower left corner. I was adding a little more shadow to make the sunrise really pop!