Filling mask layer with black

Hello!
Im new to photoshop. I was airbrushing some photos and I came across a tutorial where part of it involves creating a mask layer and filling it with black.
If you can't figure out what Im trying to say,
I need to go from this :


to this :

Hi Harsha,

On a PC you need to click on the layer mask thumbnail which is next to the thumbnail of the main image in the layers pallet. Then press CTRL + Backspace to fill a layer mask with black. ALT + Backspace will fill it with white.

Thank you George! That had me puzzled for a while!